Aug. 31: TV Ratings May Be Leading Indicator of Convention Bounce

By NATE SILVER

It remains too early to tell exactly what effect the Republican National Convention has had on the polls. But television ratings are one measure that come in almost instantaneously. Ratings for the final two nights of the Republican convention were down quite a bit from 2008, declining by about 30 percent overall.

The ratings decline should not really be a surprise. Whereas, in 2008, Senator John McCain announced his running mate, Sarah Palin, just a few days before the convention, making her a national sensation, Mitt Romney rolled out his choice of Representative Paul D. Ryan three weeks ago, perhaps limiting the buildup to Tampa, Fla.

The convention was also shortened by, and had to compete with, Hurricane Isaac. And the security in and around Tampa was airtight, limiting protests and distractions - but perhaps also the spontaneity and newsworthiness of the event.

More important, this election has simply not gen erated the same excitement from viewers and voters than 2008 did. It would be quite surprising to me if the Democratic convention did not also experience a significant decline in its television ratings.

Nevertheless, television ratings may be of some value in predicting the size of the bounce a candidate gets from the convention in the polls.

In the chart below, I've compared the historical TV ratings for the challenging party's convention against the bounce that its nominee received in the polls. Ratings are measured by the share of American households that were tuned into the convention during prime-time viewing hours.

The conventions that produced double-digit bounces for the challenger - 1968, then each year from 1976 through 1992 - were usually associated with TV ratings above 20 percent. By contrast, ratings below 20 percent have been associated with single-digit bounces, and ratings below 15 percent smaller bounces still .

It's hard to tell where the causation lies in this relationship. Do fewer voters switch their voting preferences because fewer of them watched the convention on television? Or do fewer voters feel the need to watch the convention in the first place if more of them have already made up their minds?

Regardless, the mediocre ratings for the convention are one sign that Mr. Romney's convention bounce could be fairly modest. Our forecast model will treat it as a favorable sign for Mr. Romney if his convention bounce is larger than four percentage points, and an unfavorable sign if it is under that threshold.

The Convention Bounce So Far

As for direct evidence of Mr. Romney's convention bounce from the polls, the data is pretty incomplete so far.

The most optimistic number for Mr. Romney comes from Ipsos, which has been conducting an online tracking poll on behalf of Reuters. In that poll, which includes some interviews from after Mr. Romney's accep tance speech on Thursday night, Mr. Romney now leads President Obama by one percentage point among likely voters - a five-point swing in the poll from the benchmark survey that Ipsos conducted before the convention, which had Mr. Romney trailing by four points.

The Rasmussen Reports tracking poll now has Mr. Romney ahead by one point, improved from a two-point deficit in the last poll it released before the convention. However, there are two caveats to this poll. First, very few of the interviews in the Rasmussen poll were conducted after Mr. Romney's acceptance speech on Thursday night, which means that he'll have the potential to gain more ground in the coming days. But second, the two-point lead that Rasmussen Reports had given to Mr. Obama before the convention was a bit unusual for them. The poll has had a Republican lean this cycle and has more often than not shown Mr. Romney ahead. So the movement toward Mr. Romney thus far could reflect reversion to the mean ra ther than any effects of the convention.

Finally, the Gallup national tracking poll shows Mr. Obama one point ahead: a fairly good result for him, since that poll has also had somewhat Republican-leaning results this cycle. But Gallup's polling average is calculated over a seven-day window, which means that most of its interviews still predate the convention.

For the time being, I'm feeling pretty good about my guess that Mr. Romney's bounce will come in somewhere in the neighborhood of four percentage points. It was a smartly scripted and competently executed convention (Clint Eastwood's cameo aside) but also not one that packed a lot of excitement or surprise value. In other words, it felt pretty much par for the course given how conventions are run these days.

If Mr. Romney gets a four-point polling bounce, we'd expect him to lead Mr. Obama by about two percentage points in national polls conducted between Tampa and Charlotte, N.C., since he had trailed Mr. Obama by about that margin before the Republican convention began.

The forecast model has been a bit bullish on Mr. Obama's chances lately. It now gives him a 72 percent chance of winning the Electoral College, his highest figure since Aug. 12. However, that mostly reflects some relatively favorable economic reports from among the economic data series that we track rather than anything having to do with the convention.

Delaware: A Small Example of a Larger Trend

By MICAH COHEN

We continue our Presidential Geography series, a one-by-one examination of the peculiarities that drive the politics in all 50 states and the District of Columbia. Here, a look at Delaware, the First State. FiveThirtyEight spoke with Dr. Joseph A. Pika, a professor in the Department of Political Science and International Relations at the University of Delaware.

Senator John James Ingalls, who represented Kansas from 1873 to 1891, is said to have described Delaware as a “state that has three counties when the tide is out and two when it is in.”

Mr. Ingalls's point, that Delaware is very small, is as true today as it was then. At just under 1,950 square-miles, Delaware is the second smallest state by land area. It is about 1/40th the size of Mr. Ingalls's home state of Kansas, the 15th biggest state.

Despite its small stature, Delaware has undergone the same political realignments as larger states in the Northeas t, and those changes are easier to see in a state which has just three, fairly distinct, counties (no matter what the tide is doing).

In the latter half of the 20th century, Delaware was a swing state, even a national bellwether in presidential elections. It voted for every winning candidate from 1952 through 1996. In the 1988 election, Delaware favored George Bush, a Republican, by 12 percentage points, more than his seven-point margin nationally. Now, however, Delaware is a solidly blue state. President Obama is a virtual lock to carry its three electoral votes.

In the 1990s - as the Republican brand became more defined by cultural issues like abortion and, more recently, same-sex marriage - Republican support began to erode in Delaware, where the middle of the road tends to be preferred. According to Gallup's State of the States survey, 41 percent of respondents in Delaware described themselves as moderate, the highest rate i n the nation. Many of these voters began to leave the Republican Party as it moved to the right. It is the same trend that made reliably Republican New Jersey a reliably Democratic state.

The exodus of moderate Northeastern Republicans from the party was one prong of a two-pronged realignment that essentially flipped Delaware's partisan landscape, Mr. Pika said.

At the northern end of the state is Wilmington's New Castle County, the most urbanized area in Delaware. About half of the companies in the Fortune 500 are incorporated in Delaware, where the legal framework is seen as corporate friendly. The Wilmington area is home to thousands of bankers, lawyers and other white-collar, business-focused voters who tend to hold moderate views on social issues. When they stopped voting Republican, New Castle County went from politically competitive to overwhelmingly Democratic territory.

At the same time, Delaware's southernmost county, Sussex, moved in the opposite direction. Sussex County is mostly rural, mostly white and culturally conservative. And Sussex County voters who were traditionally Democrats became reliably Republican.

Democrats in southern Delaware “were like those in South Carolina or Mississippi, ” Mr. Pika said, “They moved into the Republican Party in large numbers as the party became more connected with the Christian Right and social issues.”

The transformation has been complete. Sussex County is now Tea Party territory. It was ground zero for the 2010 Tea Party rebellion in Delaware, when conservative voters fueled Christine O'Donnell's upset victory over the establishment-backed candidate, Representative Michael N. Castle, in the Republican primary for the Senate.

Almost all of Sussex County is rural. It is the top poultry producing county in the country. Along its coast, however, more than two decades of investment has cultivated a string of resorts, Mr. Pika said, and now liberal pocket s can be found there. Rehoboth Beach, for example, has a substantial gay community.

Sandwiched between dark blue New Castle County and ruby red Sussex County is Kent County, the most politically competitive region of the state. Kent County is home to Dover Air Force Base, and many military families have settled in the area. Former President George W. Bush carried Kent County in 2000 and 2004. But New Castle's suburbs have slowly been expanding south into Kent, Mr. Pika said, moving it closer to the ideological center. Mr. Obama won Kent by eight percentage points in 2008, although much of the Democratic improvement in Kent - and Delaware generally - can be attributed to the presence on the ticket of its native son, Vice President Joe Biden, on the ticket.

The Bellwether: There Is None

Delaware does not really have a county that tends to match the statewide two party vote shares. New Castle is more Democratic than the state as a whole. Kent County is moderat ely more Republican. Sussex is a lot more Republican.

The Bottom Line

Delaware might not have a bellwether county, but political buffs wanting to know what's happening in the First State need only focus on New Castle County. With almost two-thirds of the vote, New Castle's political preferences tend to overwhelm the wishes of Delaware's other two counties. Republican presidential candidates carried both Kent and Sussex Counties in 1992, 2000 and 2004. Yet, they still didn't come close to carrying the state.

That's unlikely to change. The Democratic margin of victory in New Castle County has increased in every election since 1984, and Gallup ranked Delaware as the sixth most Democratic state by party affiliation. One in five Delawareans is black, the eighth-highest share in the nation.

On top of Delaware's default Democratic lean is the home state increase produced by Mr. Biden, who is still popular there, Mr. Pika said. It all virtually guarantees th at Delaware will be one of the first in Mr. Obama's victory column.

Aug. 30: Remember the Economy?

By NATE SILVER

We are in a holding pattern as we await polls that fully reflect the effects of the Republican National Convention.

But from the standpoint of our forecast model, the most important points on Thursday were economic rather than rhetorical. Two of the seven data series that make up the model's economic index were updated on Thursday, and they both had somewhat better news about the condition of the economy.

One of these is real personal income, versions of which are common in election forecasting models. The income numbers were extremely poor throughout 2011 and have been one of the better reasons to bet on President Obama being defeated.

However, the income figures have picked up so far in 2012, and that trend continued when the government released its July estimates of income growth on Thursday. Income grew at an annualized rate of 3.7 percent in July, according to the government's initial estimates. This closely m atches the pattern throughout the first seven months of 2012; personal income has grown at an annualized rate of 4.2 percent since January, adjusted for inflation.

The other measure is personal consumption expenditures, which make up about 70 percent of gross domestic product. The growth in consumption has been extremely sluggish over the past year - perhaps reflecting poor consumer confidence. But the July numbers were considerably better, with consumption growing at an annualized rate of 5 percent.

These measures do not receive as much attention as job growth or gross domestic product, but they are broad-based and reflect some of the many ways that voters experience and participate in the economy. They point toward what could be a stronger second half of 2012 for the economy, after a discouraging second quarter during which G.D.P. grew by just 1.7 percent.

As a result of the new economic data, Mr. Obama's chances of winning the Electoral College improved slightly in Thursday's forecast, to 71.6 percent from 70.4 percent on Wednesday.

Already a Convention Bounce?

By contrast, Mr. Obama's standing declined in our “now-cast” on Thursday - our estimate of what would happen if the election were held today. The “now-cast” looks only at polls, ignoring any economic data.

In addition, the “now-cast” does not adjust for the fact that polls usually overrate the standing of a candidate in the midst of a “convention bounce.” (The Nov. 6 forecast builds in an adjustment for these effects.)

No polls have yet been released that reflect the public's reaction to Mitt Romney's acceptance speech on Thursday, but some sought to gauge reaction to the first two days of the Republican convention.

Mr. Romney has received encouraging news from the polling firm Ipsos, which has run an online tracking poll during the convention. On Thursday, Mr. Romney led by two percentage points among likely voters in th at poll, a six-point improvement from Ipsos's preconvention benchmark, when Mr. Romney trailed Mr. Obama by four points.

The caveat is that online surveys may suffer from more selection bias than telephone polls do, perhaps reflecting the views of voters who are more politically engaged (and therefore more likely to watch the conventions) than average. In addition, Mr. Romney has shown less improvement among registered voters in the poll, with whom he still trailed Mr. Obama by three percentage points. Polls of likely voters can sometimes be more sensitive to the effects of a party convention, since one of the purposes that conventions serve is to make political partisans more enthusiastic.

Mr. Romney has also shown some improvement in the Rasmussen Reports tracking poll, which reverted to a tie on Thursday after having shown Mr. Obama ahead by two percentage points in its last poll before the conventions. Mr. Romney's position has receded slightly, however, in t he Gallup tracking poll - although Gallup reports its results over a seven-day window, so very few of its interviews yet reflect voters who watched any of the convention.

By Monday, we should have some richer data on the magnitude of Mr. Romney's convention bounce. The forecast model will treat a net gain of more than four percentage points as a bullish sign for Mr. Romney and a gain of fewer than four points as a bearish one.

In Prudent Speech, Romney Seeks Role as Generic Republican

By NATE SILVER

TAMPA, Fla. - The risk-taking Mitt Romney who picked Representative Paul D. Ryan as his running mate was not on display in Tampa on Thursday night.

Instead, in accepting the Republican nomination, Mr. Romney delivered a mostly well-written and reasonably well-delivered speech - but one that largely avoided policy substance or sweeping narrative, instead seeking to turn the election back into a referendum on President Obama.

Based on his prepared remarks, the term that Mr. Romney used most often in the speech was America, which he said 53 times.

The next most common word? Americans, which he said 35 times. Other terms that Mr. Romney used at least 10 times in the speech included jobs, better, future, business - and mom and dad. (These counts exclude common English words like the.)

But Mr. Romney uttered the word Medicare just once. He never used the terms Social Security, welfare, or entitlement.

He neve r mentioned Iraq or Afghanistan, nor did he use the term terrorism, although he did refer to the mission that killed Osama bin Laden.

Mr. Romney used the term health care just once, and Obamacare only twice. He said deficit and taxes just three times each, and budget just once. He never mentioned Mr. Obama's stimulus package, nor the federal bailouts of the financial sector and the auto industry.

Mr. Romney never used the terms gay or abortion, although he did refer to the sanctity of life and the institution of marriage. He twice referred to immigrants, but never discussed his immigration policy.

Mr. Romney only once mentioned his tenure as governor of Massachusetts, and then only to describe how he had chosen a woman as his Lieutenant Governor and appointed women to key cabinet positions.

Mr. Romney never mentioned the Olympics. He did describe his business experience at some length, although he only twice invoked the name of the company where he bu ilt his career, Bain Capital.

He even avoided some of his campaign's favorite catchphrases, like “We Built It,” the theme of the second night of the Tampa convention.

Instead, Mr. Romney's strategy was pretty clear. He was seeking to fulfill the role of the generic Republican - a safe and unobjectionable alternative with a nice family and a nice career â€" and whose main credential is that he is not Mr. Obama, the Democratic president with tepid approval ratings and middling economic numbers.

It may be a smart approach. Mr. Obama's approval ratings remain only break-even. A clear majority of voters still think the country is on the wrong track. Somewhat contrary to the conventional wisdom, the economy does not necessarily point to a defeat for Mr. Obama based on the models that political scientists and economists use to describe it - but it is not making Mr. Obama's re-election effort easy.

To the extent there is a metric that clearly reads negati vely for Mr. Romney, it is the respective favorability ratings of the candidates. Mr. Obama's ratings remain net-positive in most polls, while Mr. Romney's remain net-negative.

It was in seeking to remedy that difference where Mr. Romney's speech most broke out of its shell: first, in trying to give voters permission to vote Mr. Obama out of office even if they like him personally; and second, with a series of nostalgic and sometimes touching moments about Mr. Romney's family.

It was a speech that Mr. Romney's pollsters and consultants should have been pleased with, although it may have suffered from trying to check too many focus-group-approved boxes.

But most of all, and in contrast to Mr. Romney's selection of Mr. Ryan, it was full of the choices that a candidate makes when he thinks he can win the election by running a by-the-book campaign.

Base Turnout Strategy May Be Too Narrow for Romney

By NATE SILVER

Mitt Romney's strategy once seemed self-evident. First, skewer President Obama over the tepid economic recovery. Second, lean just enough to the center over the course of the fall campaign so you won't lose because of social issues, temperament and tone, controversial proposals about the welfare state or other things that might distract voters from your economic message.

With his selection of Representative Paul D. Ryan of Wisconsin as his running mate this month, however, Mr. Romney seems to have been pursuing a different approach, one that focuses more on turning out the Republican base.

Political messaging isn't a simple matter, and skilled political candidates can develop pitches that resonate with different audiences. In 2008, Mr. Obama simultaneously tried to brand himself as a “post-partisan” moderate, while also seeking to demonstrate to Democratic primary voters that he'd be more reliably liberal than Hillar y Rodham Clinton.

But Mr. Romney is probably a less-dexterous politician than Mr. Obama. In his acceptance speech before the Republican National Convention on Thursday night, he will probably have to tip his hand as to which goal will be receiving the most emphasis over the balance of his campaign. Will it be a speech designed to fire up the delegates in the room - and the Republican partisans who are watching on Fox News and other networks? Or more one designed to appeal to the independent voters who might be casually flipping through their cable lineups?

The argument for a base strategy is something like this: there are very few undecided voters left, and hardly anything has moved the polls. With the election being so close, the contest will come down to turnout. So get your voters as motivated as possible.

A risk for Mr. Romney, however, is that even with a favorable turnout, the Republican coalition may have become slightly too narrow for him to win, given that the party is struggling with Hispanics and other minority voters.

The pair of charts below reflect the racial voting breakdowns in 2004 and 2008, according to the national exit polls in those years. (I have made very slight adjustments to the numbers so that when you sum up the rows, they exactly match the overall percentage of the popular vote received by each candidate.)

In 2008, Mr. Obama benefited from minority voters in two ways. First, they turned out in greater numbers, making up about 26 percent of the electorate rather than 23 percent in 2004.

But what was more important is simply that a higher share of the minorities who did turn out to vote picked Mr. Obama as their candidate. His share of the nonwhite vote improved to 80 percent from Mr. Kerry's 71 percent. By contrast, his share of the white vote was only marginally better than Mr. Kerry's: about 43 percent rather than 41 percent.

It is easy to demons trate that getting a higher proportion of minority votes was more critical to Mr. Obama's success than the turnout itself. Let's run through a couple of quick, hypothetical examples.

First, imagine that the proportion of voters in each racial category who voted Democratic or Republican was the same as in 2004 - but that turnout was the same as in 2008:

Increased minority turnout alone would not have been enough to give the Democrat the election. Instead, it would have been virtually tied, with each side receiving about 49.5 percent of the vote.

Next, consider the opposite case. Turnout looks just like 2004, with a smaller share of minority voters than in 2008. However, the vote split within each racial category follows the 2008 norms, with white voters moving slightly to the Democrat, and minority voters more strongly so. This is enough to give the Democrat a fairly clear win, by about four or five percentage points in the national popular vote:

Of co urse, Mr. Obama's share of the white vote will probably not be as strong this year as it was in 2008 (when it wasn't all that strong to begin with). There is also evidence that the Republican base is more motivated to vote than the Democratic one.

So suppose that the turnout demographics this year look like 2004, when 77 percent of the electorate was white. Furthermore, suppose that Mr. Romney receives the same proportion of the white vote that George W. Bush did in 2004.

However, we'll assume that Mr. Obama does retain one advantage from 2008. Although fewer minorities turn out, those that do vote for him in the same proportions as 2008, meaning that he gets about 95 percent of the African-American vote, and about two-thirds of the vote from Hispanics, Asians and other racial minorities.

These assumptions yield a very close election - but Mr. Obama wins the popular vote. Specifically, he wins it by about 1.7 percentage points.

Interestingly, that is almost exactly the margin by which Mr. Obama leads Mr. Romney among surveys of likely voters right now.

This may not be a coincidence. The consensus of polls suggests that minority turnout may be down a bit from 2008. It also suggests that Mr. Romney should improve on Mr. McCain's numbers among white voters. However, the surveys show Mr. Romney struggling with minority voters - including Hispanics, among whom Mr. Obama maintains about a two-to-one advantage in recent surveys.

Turning out your base may not be a sufficient strategy if your base has become too narrow. In 2004, Mr. Bush had an excellent base turnout - but he also captured about 40 percent or 45 percent of the Hispanic vote, a share that Mr. Romney is unlikely to reach. Without that relatively strong performance among Hispanics, the election would have been a tossup.

None of this, obviously, is an absolute constraint. If Mr. Romney not just matches but exceeds Mr. Bush's 2004 performance among w hite voters, a base turnout strategy could be enough for him to win.

But pay attention during Thursday night's speech to whether Mr. Romney offers any moderation on issues like immigration. Otherwise, he may have drawn a very narrow path for himself between now and November.

How to Propose the Pogue Way

Two weeks ago, I proposed. To the sweetest, funniest, wisest woman I've ever known.

I wanted my children to feel a part of this life-changing event, so one night last spring, I asked them for their suggestions for a fantastic proposal. My two teenagers informed me that the most epic and unforgettable proposal would be a fake movie trailer. It would start out like any other romantic-comedy preview, but gradually reveal itself to be a thinly veiled version of our love story.

I persuaded the movie theater at a summer resort to slip it in among the real movie previews, on a night when both Nicki's family and mine were in the audience. You can watch the finished product here.

After its “premiere,” people kept asking if I'd be posting it online so they could show their friends. With Nicki's encouragement, I did - and to my astonishment, the video went viral. It also generated a lot of questions.

Here, then, are the answers.< /p>

Q. Nice job on Pogue's Proposal. But not all of us have thousands of dollars to spend on making full-blown movies for our girlfriends.

A. I had to laugh at this one. If you only knew how cheaply that movie was made! The whole thing was shot in two days, with a huge amount of planning but practically no budget.

If it looks professional, that's a testament to the skill of Zach Kuperstein, the recent N.Y.U. film school grad I hired as cameraman.

He used an ordinary Canon S.L.R. to record the video. That's it - a still camera. No Hollywood gear, no key grips or best boys. We used special lighting in only one shot, and it was the cheapie video light I wrote about last year.

I also paid the two stars - both talented Broadway actors - but all the others involved were friends and neighbors who just wanted to help out. I wrote, directed and edited the movie, and I gave myself a huge discount.

Q. Wait a minute - what about the opening shot? You clea rly had a crane rig for that.

A. Nope. Zach stood on the roof of my Prius with his tripod as I drove slowly along the street. The resulting footage was a little bumpy from the pavement, but the Analyze for Stabilization feature in Final Cut Pro X smoothed it out quite nicely.

For nighttime scenes, Zach duct-taped blankets over the windows. For the scene where “I” am hosting a Nova TV show, look closely: the “cameraman” is actually holding a 1985 VHS camcorder I found in the basement.

We didn't have a dolly, either. To film the jogging scene, Zach tied his tripod into the back of my car, like this:

As the car rolled, I sat next to him and held the microphone “boom” - a broom handle with an audio recorder duct-taped to the end of it. (For the second day of shooting, I hired a sound man with an actual boom.)

Q. In the YouTube version, we can see your girlfriend's reaction as she watches, picture-in-picture style. How could she not know she was being filmed?

A. I hid a Nikon S.L.R. in a ficus plant just beneath the screen. After the movie began, a friend moved a Sony NEX-5, on a tripod, to the back of the aisle, to record the actual proposal moment. And my daughter sat behind and across from Nicki, filming her from the side with an old Flip camera.

I was hoping that, despite the darkness and lack of rehearsal, one of those cameras would capture something usable.

Incredibly, all three did.

Q. At the end, you stand up in front of the movie audience and pop the question - and the characters on the screen seem to react. When she says “yes,” for example, they all start cheering. How could you be sure that the timing of her live reaction would work with what you'd already filmed?

A. I couldn't!

The onscreen characters interact with the live me four times. First, they encourage me to get out of my seat and bring Nicki to the “stage.” Second, when I lose my nerve, they ho ot for me to continue. Third, they gasp when I bring out the ring. And finally, they explode into cheers when Nicki says yes.

To film the on-screen sequence, I had the cast look out at a smiley face I'd taped to the tripod leg - a stunt double for where I'd be standing in the actual theater. To make sure their interruptions came at the right times, the cast reacted to the audio playback of my speech, which I had pre-recorded on my phone.

Then I spent a week rehearsing my live speech with that video, so my own timing would be right. As long as Nicki didn't say anything while I spoke, I would be golden.

Until, that is, I actually asked the question. At that point, timing was no longer in my control.

I wasn't actually worried that she'd say no; she'd dropped a few sweet hints over the months. The real problem was that I had left her enough time to say exactly one syllable.

What if she made a joke (“I'll have to think about it!”)? What if she we re too overcome to speak? What if I was too overcome to speak?

In all of those cases, the music would swell anyway, the actors would cheer inappropriately - and I'd look like the world's biggest idiot.

I worried about that moment for weeks. There was nothing I could do but hope.

As you can see in the video, my prayers were answered. Yes, I had left Nicki the time to utter one syllable - but that's all she needed.

She said “YES!”

Aug. 29: So Much Depends Upon Ohio

By NATE SILVER

Polls that are published during the party conventions can be anticlimactic, representing old news since they won't fully reflect the effects of the convention bounce. And none of the surveys that came out on Wednesday were that newsworthy to begin with â€" although there was one, in Ohio, that had encouraging news for Barack Obama, and another, in Nevada, that was slightly favorable for Mitt Romney.

The Ohio survey was conducted by the automated-polling firm Gravis Marketing, which had Mr. Obama ahead of Mr. Romney by less than 1 full percentage point.

A one-point lead isn't much, and Mr. Obama has gotten some better numbers than that in Ohio. So why does this qualify as good news for him? Because this firm has had Republican-leaning results in the other states that it has polled, putting Mr. Romney up by 2 points in Florida, 1 point in Colorado and 17 points in Missouri, making it several points more Republican-leaning than the consensus of surveys in those states. Once the model adjusts for the firm's “house effect,” it treats Mr. Obama's nominal 1-point lead as being the equivalent of a 4- or 5-point lead instead. Thus, Mr. Obama's chances of winning Ohio rose somewhat based on the survey.

The broader point is simply that Ohio is so important to the electoral calculus that it's good news for a candidate when a polling firm shows him doing relatively well there compared with the other states that it polls. Ohio has a 30 percent chance of being the tipping-point state, meaning that it would cast the decisive votes in the Electoral College. That's as much as the next two states on the list, Florida and Virginia, combined. It's also as much as Colorado, Nevada, Iowa, Pennsylvania, New Hampshire, Michigan and North Carolina combined.

All of these states are competitive. But really, they exist along a continuum of electoral power rather than falling into binary categories of †œimportant” and “unimportant.” Ohio is at the extreme end of that continuum.

The reason our tipping-point calculus rates Ohio so highly is because it would usually suffice to provide Mr. Obama with a winning map, even if he lost many of those other states. If you give Ohio to Mr. Obama, plus all the states where the forecast model now estimates that he has at least 75 percent chance of winning, he's up to 265 electoral votes. That means he could win any one of Colorado, Virginia, Iowa, Wisconsin, Florida or North Carolina to put him over the top.

Mr. Romney is fortunate in this sense to have put Wisconsin squarely into play with his selection of Paul D. Ryan as his running mate; it gives him a few more ways to win without Ohio, although it would still be a daunting task.

…But Don't Forget Nevada

Another state that could be critical to an Ohio-less winning path for Mr. Romney is Nevada. I did not mention it in the list above since the model giv es Mr. Obama exactly a 75 percent chance of winning there. But Mr. Romney's numbers have been a bit better in Nevada lately, including in a Public Policy Polling survey, released on Wednesday, that put him behind Mr. Obama by 3 points there.

Obviously, Mr. Romney would prefer to be ahead in Nevada than behind by any margin. But that is a better result for him than the previous Public Policy Polling survey of Nevada, which put him down by 6 points instead. The firm's surveys are also somewhat Democratic-leaning, although they have been moving closer to the consensus recently.

The pessimistic case for Mr. Romney in Nevada is that the polling spread has been narrow there compared with other states. In the 12 Nevada surveys in our database, Mr. Obama has never trailed â€" but nor has he led in any survey by more than 8 points. A tighter spread in the polls makes a smaller lead more robust, as does the fact that there are very few undecided voters in the Nevada polls.

Still, Nevada is a state that has produced some poor polling in the past, and between a potentially strong Mormon turnout and the state's dilapidated economy, Mr. Romney has some angles to work there.

Measuring a Convention Bounce

By NATE SILVER

Polling generally becomes more accurate as you get closer to Election Day. The exception to the rule is in the period immediately after the party conventions, when the polls can be a wild ride.

In some past elections, candidates have received as much as a 25-percentage-point lift in the polls after their convention. A bounce of that magnitude is unlikely this year; in fact, there is reason to think that the change in the polls will be quite modest. Nevertheless, this will make poll-reading more difficult.

The FiveThirtyEight forecast will build in an adjustment for the effects of the conventions. It's possible that a candidate could gain ground in the polls but lose ground in the forecast if his bounce is below average. The model will take it as a bad sign for Mitt Romney, for instance, if he fails to pull ahead of President Obama in polls conducted in the brief window between the Republican and Democratic conventions - even if he gains a point or two.

Historical Convention Bounces

Let's start by looking at how the polls progressed in past election cycles in the period surrounding the conventions. (We'll go back to 1968, which is when our polling database becomes more robust.)

The way we look at this data is going to be a bit different than we do ordinarily. Instead of thinking of the candidates as Republicans or Democrats, we'll break them down into incumbent and nonincumbent groups.

For these purposes, a candidate counts as the “incumbent” if his party controls the White House at the time of the election, even if he is not the president himself. So, for example, in 2008, John McCain is listed as the incumbent candidate since another Republican, George W. Bush, was president at the time, and Mr. Obama as the challenger.

In the charts that follow, the letter “I” and the color black designate the incumbent party; the letter “C” and the color orange designate the challenger. During this time period, the challenging party always held its convention first. (The challenging party is sometimes also designated as the “out-party” - as opposed to the incumbent party, which is also called the “in-party.”)

For each election cycle, I've taken an average of the national polls in our database during five different periods surrounding the conventions. I'm presenting this data in two slightly different ways.

The first table simply reflects the polling average. So, for example, the designation “C +5″ indicates that the challenger was ahead by five points, on average, in polls conducted during that period.

The second version of the table evaluates the bias in the polls, comparing the polling during the convention period against the actual election result. If you see the designation “C +5″ in the second chart, what that means is that the challenging candidate polled five poi nts higher at that time than his actual finish on Election Day. In other words, the polling was biased toward him by five points because of the convention bounce.

All numbers are taken on a net basis: they reflect the margin between the candidates. If the Democrat improves to 47 percent from 43 percent in the polls after his convention (a four-point gain), and simultaneously, the Republican declines to 49 percent from 53 percent (a four-point loss), we'd describe that as an eight-point bounce since the Democrat went from being a net of 10 points behind to just two points behind instead.

The first column in the charts, Column A, looks at the polls in the three weeks immediately preceding the challenging party's convention. Since the challenging party always held its convention first, this gives us a sense for what the polling baseline was before the conventions. In 6 of the 11 years in our study, the challenging party held the polling lead before the conventions, although it was modest in most of these cases.

The next time frame in Column B is more critical: it consists of the polls in the two weeks immediately after the challenging party's convention (but before the incumbent party's convention). This is when it would ordinarily be at the height of its convention bounce.

The only challengers who still trailed immediately after their conventions were George McGovern in 1972 and Bob Dole in 1996 - even Walter Mondale, in 1984, had somehow managed to pull into a tie with Ronald Reagan.

Almost all of the challenging candidates, also, polled higher during their convention bounce than they eventually realized on Election Day. The exceptions? Mr. Obama in 2008, who got a small convention bounce and was ahead by only about four points after the Democratic convention in Denver that year. He eventually won the election by seven percentage points instead. And in 1996, Mr. Dole trailed by nine percentage points after his conve ntion, the same margin by which he eventually lost to Bill Clinton.

On average, however, the polls just after the challenging party's convention overrated their standing by a whopping 14 points relative to their actual finish. Even candidates who got a lot of momentum out of their conventions, like Mr. Clinton in 1992 and Mr. Reagan in 1984, saw inflated polling during this time period.

The third column, Column C, consists of an interim period: those polls conducted more than two weeks after the challenging party's convention, but before the incumbent party's. This category does not apply in cases where the conventions were separated by two weeks or less.

In all years where this data is available, the bounce had faded somewhat after a couple of weeks. But it hadn't faded completely, as six of the seven challenging candidates still polled better during this period than their Election Day finish.

The fourth category, Column D, is an average of polls in the two weeks right after the incumbent party's convention. The incumbent party led after its convention in 7 of the 11 years; it trailed in 1968, 1976 and 1992, and Jimmy Carter and Mr. Reagan were roughly tied after the Democratic convention in 1980.

But it gets a little tricky when we want to describe the size of the incumbent party's convention bounce, since it's not clear what baseline we should be measuring it against.

As compared with where the polls stood at the peak of the challenging candidate's convention bounce, the incumbents had gained back a lot of ground - 15 percentage points on average.

However, they had gained much less ground - only three or four percentage points - relative to where the polls stood before either party's convention.

Moreover, the polls conducted during this period did not exaggerate their standing all that much. The incumbent was ahead by an average of five percentage points in polls conducted just after his conven tion, but eventually won the election by an average of three points, only a modest difference.

This may simply reflect the fact that the incumbent's convention is often held at a time when the challenger's convention was recent enough to still have some effect on the numbers. That is, the incumbent's convention bounce is partly canceled out by the aftereffects of the challenger's. By contrast, the challenger gets a window of time when he pretty much has the stage to himself, until the incumbent holds his convention.

Finally, Column E consists of the polls in the period two to five weeks after the incumbent party's convention, when the effects of the convention bounce should be fading.

The polls during this period are not that much different from those in Column D. Nor were they much different from the eventual election results on average. The biases that the conventions introduce into the polls seem to have worked their way out of the system by this point i n time.

Does Convention Bounce Predict Election Results?

So it's really the period just after the challenger's convention, but before the incumbent's convention, when the polls can be the most skewed.

Do the polls conducted during this time frame have any predictive value? Or are they so far from the mark that it's better to stick with preconvention polls, then wait until both parties have had their conventions before you start paying attention again?

Actually, the answer is a little unclear.

If you run a regression analysis, it suggests that the postconvention polls probably have a bit of predictive power, but ought to be discounted fairly heavily. You should still be looking at the preconvention polls as well.

However, it's hard to tell because the postconvention and preconvention polls are fairly highly correlated with each other. When variables are highly correlated, regression analysis is less useful, especially with only 11 elections to evaluate to differentiate between them.

On a more anecdotal level, there have certainly been cases, as in 1980 and 1992, when the challenging party got a huge convention bounce and rode to victory. But there are also exceptions to the rule: Mr. Mondale's fairly big bounce in 1984, which was very short-lived, for instance. In 1988, Michael Dukakis was already ahead in the polls before the conventions and then got a pretty big convention bounce, but wound up losing. In 2008, conversely, Mr. Obama got a rather small convention bounce - Mr. McCain and Sarah Palin got a bigger one - but Mr. Obama ended up on top.

Is the Size of the Convention Bounce Predictable?

In this section, I'm going to focus on the challenging party's convention bounce. As I mentioned, it is much more straightforward to evaluate the challenger's bounce than the incumbent's, since what we think of as the incumbent's bounce may really just reflect the challenger's bounce fading. Are ther e patterns in which challengers tend to get bigger or smaller bounces?

One hypothesis is that the challenger tends to get a bigger bounce when the economy is poor. But in my view, that's not really reflected in a consistent way in this data. Mr. Clinton in 1992 and Mr. Reagan in 1980 got huge bounces amid a bad economy. But Mr. Obama got very little bounce despite a bad economy in 2008. And some challengers, like Mr. Mondale in 1984 and Mr. Carter in 1976, got big bounces despite a good economy. There just doesn't seem to be much of a consistent relationship.

Another theory is that the challenger tends to get a bigger bounce when he is less well known to voters. This idea fits well with the examples of Mr. Carter in 1976 and Mr. Clinton in 1992, both of whom were fairly obscure figures before winning the Democratic primaries that year, and both of whom got very large bounces.

But there seem to be just as many exceptions. Richard Nixon in 1968, and Mr. Monda le in 1984, were extremely well known to voters in those years, having been former vice presidents. Both got pretty big bounces despite being known commodities. John Kerry, in 2004, was not all that well defined to voters before the Democratic convention that year, but he got almost no bounce despite a lot of emphasis on his biography at that convention.

One thing that seems to be clearer is that the convention bounces are getting smaller. Mr. Kerry and Mr. Obama got very small bounces. And the challenging candidate in 2000, George W. Bush, got only a modest-size one.

By contrast, six of the seven challengers between 1968 and 1992 got double-digit bounces, the exception being Mr. McGovern in 1972.

This likely reflects some ways in which the political climate is changing. With news coverage being what it is, we are living more and more in a “perpetual campaign.” There is certainly a decent share of voters who will start paying a lot more attention to th e campaign once the conventions take place. But it's not going to be quite as night-and-day as it was 20 or 30 years ago, before the advent of the Internet and 400-channel cable lineups.

Perhaps more important, partisanship has steadily increased during this period. There are fewer and fewer true swing voters, and it is harder to move the polls for any reason, with so many people locked into voting Democrat or Republican from the get-go.

Stable Polls, Smaller Bounces

In fact, there is a relationship between how much the polls move before the conventions and the size of the convention bounces. The more stable the polls are before the conventions, the smaller the magnitude of the convention bounce.

In the chart below, I've listed the standard deviation of all national polls conducted between November of the pre-election year, and the challenging party's convention. The higher this number, the more volatile the polls are. Then I've compared it against th e size of the challenging party's convention bounce.

The relationship is not perfect, but it's reasonably strong, explaining about 40 percent of the variance in the convention bounce. The four years with the most volatile polling before the conventions - 1980, 1976, 1988 and 1992 - all produced large convention bounces. The two years when the polling was the most stable, 2000 and 2008, produced fairly small ones.

But what about this year? The national polls have been exceptionally stable, dating back to late 2011. Except for a period in February and March when Mr. Romney was struggling in the Republican primaries, they've pretty much always showed something on the order of a two-point lead for Mr. Obama.

In fact, as measured by the standard deviation of national polls since November, this has been the least volatile polling year ever. The standard deviation in the national polls has been 3.7 percentage points, as compared with a historical average of 6.7 po ints.

What this means is that we should probably expect fairly small convention bounces. The polls have remained quite steady through events like the Supreme Court's ruling on Mr. Obama's health care bill, various discouraging and encouraging economic reports, and Mr. Romney's selection of Representative Paul. D. Ryan of Wisconsin as his running mate. The conventions will probably produce some effect, but it would be an upset if it were all that dramatic given how immovable the polls have been so far.

Specifically, the benchmark is for a convention bounce of about four points for each candidate. Since Mr. Romney trails Mr. Obama by about two percentage points now, this implies that he'd move ahead of him by about two percentage points instead in polls conducted immediately after this week's convention in Tampa, Fla.

If Mr. Romney gets a larger bounce than that - say, pulling ahead of Mr. Obama by perhaps four to six points in the national polls - that would be a bullish sign for him. This wouldn't reflect all that large a bounce as compared with the historical average. However, it would be pretty impressive given how hard it has been for the candidates to move the numbers for any reason at all this year.

By contrast, if Mr. Romney only pulls into a tie with Mr. Obama, or still trails him in polls conducted this weekend, that would be a troubling sign for his campaign. The track record of challengers who failed to lead after their conventions is not good.

When Two Bounces Overlap

In 2008, I modeled the convention bounces as a function of the number of days that had passed since each party's convention began. The convention bounce seems to peak a day or two after the party finishes its convention, then fades slowly but steadily.

This year, I've modified the analysis to reflect the factor that I described above: that we expect smaller-than-average convention bounces because of the low volatility of the pol ling up to this point in time.

However, this is further complicated by the fact that there are only a few days separating the party conventions this year. We'll barely have had time to benchmark Mr. Romney's convention bounce before the Democrats start their convention in Charlotte, N.C., which could counteract its effects.

Our approach to this problem is to assume that each candidate will get a convention bounce of the same size and shape. (It once was the case that the challenging party seemed to get a larger bounce than the incumbent party, but that wasn't true in 2000, 2004 or 2008.) Those bounces might look something like this:

Note, however, that the bounces overlap with each other. We'd expect Mr. Romney to still be experiencing some of his convention bounce at the same time that Mr. Obama begins his.

If you take the superposition of the two convention bounces - meaning, essentially, that you add them together - you wind up with a funky looking chart like this:

We'd expect the polls to initially move by about four points toward Mr. Romney this weekend, reflecting the effects of the Republican convention. However, the bounce would be short-lived, since the Democrats will begin their convention on Tuesday.

Then Mr. Obama gets his convention bounce. However, it would be smaller than Mr. Romney's since not all that much time will have passed since the Republican convention, canceling it out in part.

The benchmark is that we might expect Mr. Obama's polling just after Charlotte to be one or two points better than it was before either party's convention, meaning that he'd be ahead in national polls by three or four points.

This set of assumptions squares pretty well with the historical data, in which the challenging party gets a fairly clear bounce, but the incumbent party's is muted. Still, it would be a poor sign for Mr. Obama if he failed to lead in the polls after his convention in Charlotte.

Adjusting for the Convention Bounce

As I mentioned, the FiveThirtyEight forecast will adjust for the convention bounces.

Mathematically, this adjustment is not complicated: we'll just subtract out the projected effects of the convention from the polls.

So, for example, if Mr. Romney holds a five-point lead in a poll conducted this weekend, the model will instead treat that as a one-point lead. This is because this should be right in the midst of his convention bounce and because our benchmark for his convention bounce is four points this year.

This also implies that if Mr. Romney does not gain any ground in the polls after this convention, the model will treat that as a significantly negative factor for him. Then Mr. Romney will have to hope that Mr. Obama does not get any convention bounce either.

I know that some readers will not like the convention bounce adjustment. But it's critical to remember that a convention bounce is almost alwa ys temporary. Sometimes, the bounce will be large enough that a candidate will go from a losing position to a winning one even after it fades, as Mr. Clinton did in 1992. But the polls almost always overrate a candidate's standing in the interim.

We do hedge against the convention bounce adjustment in two ways, however. First, in addition to adjusting the polls during the convention period, we'll also assign them less weight. (Specifically, polls conducted when the absolute value of the convention bounce is at its highest will receive a 50 percent penalty, with everything else scaled accordingly.)

More important, we have an alternative for those of you who prefer to look at the polls on an “as-is” basis. That is what we call our “now-cast,” which is our estimate of what would happen if the election were held today. We give the now-cast less emphasis than our forecast, but you can find it by clicking the “now-cast” tab in the right-hand column of this page.

The now-cast will not apply the convention bounce adjustment. It's possible that one of the candidates could pull ahead in the now-cast while losing ground in the forecast, if he gets some sort of a convention bounce, but it's a below-average one.

The now-cast also differs from the forecast in other ways. It does not include any economic variables, as the forecast does. It weights the most recent polls somewhat more heavily. And it has a smaller margin of error associated with it, because there is less uncertainty about what would happen in an election held today than the one that will take place in November. But it should give you more options for how you evaluate our data.

Still, the forecast is our signature product. We expect the convention bounces to be small this year. But if Mr. Romney gets no convention bounce at all, or a bounce of only one or two percentage points, it will be appropriate to take a more pessimistic view of his chances of winn ing in November.

In Florida, Tampa Is Essential to Romney Election Hopes

By MICAH COHEN

We continue our Presidential Geography series, a one-by-one examination of the peculiarities that drive the politics in all 50 states and the District of Columbia. Here, a look at Florida, the Sunshine State. FiveThirtyEight spoke with Lance deHaven-Smith, a professor of public administration and policy at Florida State University, and Daniel A. Smith, a professor in political science at the University of Florida.

The Republican Party has good reason to hold its national convention in Tampa, Fla. The Tampa area is the most competitive section of the most competitive region in one of the most competitive states in the nation - the perfect place to seek a glimmer of extra advantage in a closely-fought presidential contest.

In many ways, the Tampa area was the weakest link in the regional coalition that Barack Obama built to win Florida in 2008. The Tampa-St. Petersburg media market is home to a quarter of Florida's registe red Republicans, and Mr. Obama carried Hillsborough and Pinellas Counties - home to Tampa and St. Petersburg - by a smaller margin than Florida's other major population centers. If Mitt Romney wants to win the state, it represents the most attractive target.

And winning Florida is a must for Mr. Romney. Based on the simulations that the FiveThirtyEight forecast model ran on Tuesday, Mr. Romney has only a 0.3 percent chance of winning the election if he loses the state.

It is hard to conceive of Mr. Romney winning the election but losing Florida because Florida is an ever-so-slightly Republican-leaning state. If he loses it, he's probably having trouble elsewhere on the map as well. It's quite unlikely that Mr. Romney loses Florida but wins a state like Michigan or Pennsylvania, for instance.

Still, the state is idiosyncratic enough that it could behave differently, moving in one direction while the other swing states move in another. Each party begins with some strengths and weaknesses in the state, and regions that are vital to its path to victory there.

Just over a third of Florida's registered Democrats live in the Miami and West Palm Beach media markets, especially in Miami-Dade County, Broward County and Palm Beach County.

Broward County, in particular, is critical to Democratic margins in Florida. While Miami-Dade County is reliably Democratic, its large Cuban-American population leans Republican and keeps the county from tilting all the way to the left. Miami-Dade County is home to 58 percent of Florida's Hispanic Republicans and 34 percent of Hispanic Democrats. Without Broward County, however, Mr. Obama would have lost Florida in 2008; his statewide margin of victory (204,577 votes) was less than his margin in Broward (252,948 votes).

Democratic support has grown more recently in Orlando, a historically Republican city and the second main population center in the Interstate 4 corridor (along with Tam pa). Orlando's Orange County was just marginally Democratic in the 2000 and 2004 presidential elections. Then - partly because of an influx of non-Cuban Hispanics - Mr. Obama carried Orange County fairly easily in 2008, and the county itself is probably out of reach for Republicans now.

“It's really tipping the state,” Mr. deHaven-Smith said. A potential dream scenario for Democrats - and a nightmare for Republicans - is if the demographic shifts in this region are enough to shift Florida from being slightly Republican-leaning to strictly neutral, or slightly Democratic-leaning instead.

The Jacksonville area has also moved to the left, although from a more conservative starting point and for slightly different reasons. That region, which has a significant military presence, is known for its culturally moderate, business-oriented Republicans, Mr. Smith said. Former President George W. Bush won Jacksonville's Duval County with 58 percent of the vote in both 200 0 and 2004.

Those same voters are still a majority, but just barely. Duval County has become more competitive as its African-American population has grown and as black turnout has increased, Mr. deHaven-Smith said. In 2008, Mr. Obama lost Duval by just one percentage point.

East of Jacksonville, Florida begins to look and feel like its northern neighbors, Georgia and Alabama. Affectionately known as the “Redneck Riviera“, the Florida Panhandle's beautiful beaches are dotted by resorts and military installations. The area is culturally conservative and heavily Republican, but most of the counties in the Panhandle are sparsely populated.

The last predominately Republican area of the state is southwest Florida, where a string of retirement communities stretch along the coast, giving the G.O.P. a solid base of reliable voters. More affluent retirees tend to live in Collier County, while more working-class retirees have settled in Charlotte County, Mr. deHav en-Smith said. But both communities, like Fort Myers's Lee County and most of the rest of Florida, were hit hard by the collapse of the housing sector.

The Bellwether

That leaves the Tampa area as neutral - but critical - turf. Of course, both Democrats and Republicans can do a little worse in one region if they do a little better in another. But in general, carrying Tampa has been the final step for statewide winners. In every election since 1960 the presidential candidate who carried Florida has also carried Tampa's Hillsborough County.

Economically and ethnically, Hillsborough County is a fairly good microcosm of America. The county's Democratic core in Tampa is balanced by the city's Republican suburbs, and recent Hispanic growth has been accompanied by exurban growth, maintaining the county's partisan equilibrium.

The Bottom Line

Florida is the closest state according to the FiveThirtyEight forecast; Mr. Obama was a nominal 52 percent favor ite there as of Tuesday, but it has flipped back and forth between the candidates many times.

Still, and particularly so long as Mr. Obama seems to hold a clearer advantage in other swing states like Ohio, it is much easier to conceive of Mr. Obama winning the election without Florida than Mr. Romney. For that reason, it represents more of an offensive state for him.

For over a year, Mr. Obama has been building an extensive infrastructure in the state, despite the state's reputation for being dominated by the advertising “air war.” The Obama campaign has opened 73 field offices in Florida compared with the Romney campaign's 28.

In Mr. Romney's favor is Florida's still struggling economy. In particular, the state's housing sector continues to flounder. Apart from the general distaste that a poor economy can cultivate for an incumbent, the Obama campaign's efforts to reach out to voters have been complicated by Florida's many home foreclosures, which have uprooted thousands and rendered voter lists inaccurate.

Ultimately, the calculus for undecided Tampa voters is the same for Florida voters, which is much the same as it is for undecided voters elsewhere: how much is the president to blame for the tepid economic recovery, in Florida and the nation? And is Mr. Romney a suitable alternative? The way voters in Tampa answer those questions could help determine the next president.

Aug. 28: A Moment of Polling Clarity

August 28, 2012, 11:44 pm

By NATE SILVER

At FiveThirtyEight, we're usually consumed by how much the polls disagree with one another, and where the race stands once you filter through all that noise. But every now and then, the pollsters make our job easy.

Over the past several days, there have been 15 national polls released. It looks like everybody has been trying to get numbers out as a baseline before the party conventions. And they tell a pretty consistent story.

Nine of the 15 polls have President Obama ahead by either one or two percentage points. Three have Mr. Obama ahead by a slightly larger margin, between four and six percentage points. Another three have Mr. Romney ahead, each by a single percentage point.

On average betwee n the 15 surveys, Mr. Obama leads by 1.6 percentage points. While there are some modest differences between them, pretty much every poll is within the margin of error of the others.

Enjoy this moment of relative consensus in the polls; it probably won't last long.

Swing State Connecticut?

To get a more controversial polling result, we instead have to turn toward Connecticut, where Quinnipiac released a poll on Tuesday showing a relatively close race. Quinnipiac had Mr. Obama by seven points in Connecticut, making this the third consecutive poll to show Mr. Romney within single digits there.

Our forecast model's view is that these surveys slightly underrate Mr. Obama's position in Connecticut; it projects that he'll eventually win the state by 10 or 11 percentage points. Still, that would be a big drop-off from 2008, when he won the state by 22 points.

It's not that hard to see how Mr. Romney could be overperforming i n Connecticut. It has lots of wealthy voters who are employed in finance, particularly at private equity firms and hedge funds, industries that have become skeptical of Mr. Obama's performance. Mr. Romney has raised more money than Mr. Obama in Connecticut, about $5.2 million in itemized contributions, against $3.5 million for the president.

Could Connecticut actually figure into the electoral math? It's obviously not likely. The model gives Connecticut just a 0.2 percent chance (about 1 chance in 500) of being the tipping point state and casting the decisive votes in the Electoral College.

Still, it's interesting to contemplate cases in which Republicans began to be more competitive again in the Northeast. In this hypothetical map, for example, the Republican candidate wins Connecticut, New Jersey and New Hampshire, enough to give him a winning map with 272 electoral votes, despite his losing most of the swing states in the West and the Midwest.

The odds a re hundreds-to-one against a map like this coming to fruition this year. But it could be more plausible in 2016 if the Republicans nominated a relatively moderate candidate who downplayed social issues - positions that have turned off a lot of voters in states like Connecticut and New Jersey who might vote G.O.P. if they were going by their pocketbooks.

In the meantime, a relatively strong performance by Mr. Romney in Connecticut could help the Republican Senate candidate, Linda E. McMahon, to carry the state. She held a lead over her Democratic opponent, Representative Christopher S. Murphy, in the Quinnipiac poll.

Ms. McMahon did not wear well with voters over the course of the 2010 campaign, and she is fighting against a lot of partisan gravity in Connecticut, which hasn't elected a Republican to the Senate since Prescott Bush in 1956. But if Mr. Romney can keep the race within single digits, she'll have a considerably better chance of holding on.

Aug. 27: Michigan Isn\'t a Tossup

By NATE SILVER

In this update, I'm going to focus on the polling we've seen in one particular state, Michigan, where there is an exceptionally wide spread between firms showing relatively strong numbers for Barack Obama, and those which have good numbers for Mitt Romney instead.

But first let me mention a poll that is a bit easier to interpret. The Washington Post and ABC News released their latest national poll on Monday, and it had favorable numbers for Mr. Romney, showing him one point ahead of Barack Obama among registered voters.

Past Washington Post/ABC News polls have often shown a pretty tight race, so this poll doesn't necessarily speak to a dramatic change in the state of play. But sometimes a more basic point gets lost when we get as detail-oriented about the polls as we sometimes do at FiveThirtyEight.

The Post/ABC poll is a very good survey, with strong methodology and a strong track record. The point is simply that it's good for a candidate when a strong polling firm shows him doing well. In any election, there are going to be polling firms that publish relatively stronger and relatively weaker numbers for a candidate; this is one of those surveys that you'd very much like to have on your side. And so far this cycle, Mr. Romney has tended to get the better results from the Post/ABC poll.

The Mysterious Polling in Michigan

There are also surveys that you don't necessarily want on your side, however, and some states seem to have a concentration of them. One of them is Michigan, where since mid-July we've seen polls showing everything from a 14-point lead for Mr. Obama in Michigan to a 4-point lead for Mr. Romney. That is a remarkably wide spread - too wide to be caused by statistical variance alone. Someone is getting this race wrong.

Our forecast model tends to side with the polls showing Mr. Obama in a reasonably good position in Michigan. It's extremely doubtful that he's ahead by double-digits there - and almost certainly not by 14 points - but the forecast projects a six- or seven-point victory for Mr. Obama in Michigan on Nov. 6. Let me explain how it comes to that conclusion.

In the table below, I've broken down the Michigan polls into two categories: those conducted by firms that frequently survey states outside of Michigan, and those that poll Michigan only, or Michigan and one other state.

The national polling firms, like Rasmussen Reports, Public Policy Polling, and Marist College, show Mr. Obama ahead by an average of seven points in Michigan. By contrast, the Michigan-only pollsters show him ahead by only two points.

The advantage of the national pollsters is that we have a good sense for how to interpret their results. Some are Democratic-leaning and some are Republican-leaning - but whatever “house effect” they tend to show, it is usually fairly consistent from state to state.

By contrast, when a polling firm takes surveys in one state only, and its results diverge substantially from the consensus, it's harder to tell what is going on. It could be that there is something about Michigan in particular they are seeing. Or it could be something more intrinsic to the methodology: that if the firm shows good numbers for Mr. Romney in Michigan, for example, it would also do so in Colorado, or Virginia, or Ohio, if only they surveyed those states.

The model is designed to mostly prefer the latter interpretation. If a local polling firm diverges substantially from the consensus, it infers that the results show a strong “house effect,” rather than providing any particular insight about the state. It doesn't entirely dismiss the poll, but it treats it skeptically.

But isn't local knowledge a good thing? It can be, and there are some outstanding state polls conducted by local newspapers or local universities, which are rewarded accordingly in our pollster ratings . However, there are also some very questionable ones.

In Michigan, there are two local polling firms in particular that have shown poor numbers for Mr. Obama. One is Mitchell Research, which just published a poll showing a tie there. The other is Foster McCollum White Baydoun, which had Mr. Romney ahead by four points in its last survey.

Foster McCollum White Baydoun actually does survey one state apart from Michigan: it released a poll of Florida last week. In that survey, it showed Mr. Romney ahead by almost 15 points - perhaps the biggest outlier we've seen in any individual poll so far this year.

As I mentioned when that Florida poll came out, this firm anticipates a big decline in participation among groups like younger voters that are ordinarily inclined to support Democratic candidates. The initial news release of the Florida poll appeared to show that the firm anticipated that just 11 percent of Florida voters would be under age 50 - in contrast to about half the Florida electorate in 2008, according to exit polls. A principal of the firm, Eric Foster, later wrote me to say that the news release had been incomplete, and voters under the age of 50 had actually been weighted to make up 27 percent of the poll instead.

That's still an implausibly large drop-off, in my view, from 2008. It may be because the Foster McCollum White Baydoun likely-voter model looks for voters who, as they described it, participated in “odd year municipal and county elections” - where turnout is tiny as compared with what it will be on Nov. 6. In any event, this method seems likely to produce extremely Republican-leaning results - whether applied to Michigan, to Florida or to any other state.

At least Foster McCollum White Baydoun have a methodology of some kind, even if it isn't one that I would recommend. What Mitchell Research is doing in Michigan is a little more troubling.

The head of Mitchell Research, Steve Mitchell , wrote a long memo accompanying his poll release on Monday. In that poll, he weighted the survey to assume that African-Americans would make up only 8 percent of Michigan's turnout. By contrast, black voters represented 12 percent of the turnout in Michigan in 2008 according to exit polls, and 14 percent according to another source, the Current Population Survey. Blacks also made up 13 percent of Michigan's vote in 2004 and 11 percent in 2000, according to exit polls. African-American participation is sometimes lower in midterm election years, but blacks were 12 percent of Michigan's electorate in 2006, the exit poll reported that year. (There was no exit polling in Michigan in 2010.)

So why only 8 percent this year? Mr. Mitchell wrote that he simply doesn't believe the exit polls:

“African-American participation in this poll is 8%, not 12%, which is the percentage of the population but not likely voters. I do not believe blacks represented 12% of the vote in 2008 and I don't believe they will in 2012. Having polled this state for 26 years, blacks have represented about 7%-8% of all votes cast in every statewide race. At best, it went to 9% four years ago. It will not reach that level this year.”

What is the evidence for Mr. Mitchell's claim? He didn't present any of it in the memo. The exit polls certainly aren't perfect, but they've been consistent from year to year and also seem to agree with Census Bureau data. (Another fact that Mr. Mitchell cites - that African-Americans represent 12 percent of the overall population in Michigan - is also slightly incorrect; they made up 14 percent of Michigan's population as of 2011, according to the Census Bureau.)

Perhaps Mr. Mitchell means that only about 8 percent of respondents are black when he takes his surveys. But response rates to political surveys have been much lower among African-Americans for many years. The problems may be worse in a su rvey, like Mr. Mitchell's, that does not call cellphone voters, since African-Americans are more likely to rely on cellphones in lieu of landline phones.

It is precisely because of this problem that essentially all polling firms do weight by race, usually calibrating their numbers based on Census Bureau or exit poll data. Mr. Mitchell, however, seems to have his own ideas about what Michigan's electorate will look like. There is a certain amount of art in political polling, but I've never heard of a pollster treating the demographic makeup of a state as essentially a matter of opinion.

Mr. Mitchell's demographic weights account for much of the difference between his polls and those of the other firms that are active in Michigan; a decrease in the African-American vote from 12 percent to 8 percent would harm Mr. Obama's numbers by a net of about four percentage points there. If you have some reason to discount the Mitchell Research and Foster McCollum White Baydou n polls, it is much harder to make the case that the presidential and Senate races in Michigan are a “tossup,” as some other Web sites have characterized it.

To be clear, our forecast model doesn't “know” all of this detail about the Mitchell Research and Foster McCollum White Baydoun polls. It discounts the results from these firms simply because that's what it's programmed to do whenever it encounters a local polling firm whose results diverge substantially from the consensus; this method works to Mr. Obama's benefit in some states, and Mr. Romney's in others.

But my personal view is that the model is making the right inference in the case of Michigan, and that the methodological choices that these polls have made are hard to defend. Perhaps 90 or 95 percent of the time, taking a simple average of the polls will work just about as well as the more complicated FiveThirtyEight method. But this is rare instance where taking all the polls at face value may be a mistake, and the additional checks-and-balances the FiveThirtyEight method applies are worth the trouble.

Aug. 27: Michigan Isn\'t a Tossup

By NATE SILVER

In this update, I'm going to focus on the polling we've seen in one particular state, Michigan, where there is an exceptionally wide spread between firms showing relatively strong numbers for Barack Obama, and those which have good numbers for Mitt Romney instead.

But first let me mention a poll that is a bit easier to interpret. The Washington Post and ABC News released their latest national poll on Monday, and it had favorable numbers for Mr. Romney, showing him one point ahead of Barack Obama among registered voters.

Past Washington Post/ABC News polls have often shown a pretty tight race, so this poll doesn't necessarily speak to a dramatic change in the state of play. But sometimes a more basic point gets lost when we get as detail-oriented about the polls as we sometimes do at FiveThirtyEight.

The Post/ABC poll is a very good survey, with strong methodology and a strong track record. The point is simply that it's good for a candidate when a strong polling firm shows him doing well. In any election, there are going to be polling firms that publish relatively stronger and relatively weaker numbers for a candidate; this is one of those surveys that you'd very much like to have on your side. And so far this cycle, Mr. Romney has tended to get the better results from the Post/ABC poll.

The Mysterious Polling in Michigan

There are also surveys that you don't necessarily want on your side, however, and some states seem to have a concentration of them. One of them is Michigan, where since mid-July we've seen polls showing everything from a 14-point lead for Mr. Obama in Michigan to a 4-point lead for Mr. Romney. That is a remarkably wide spread - too wide to be caused by statistical variance alone. Someone is getting this race wrong.

Our forecast model tends to side with the polls showing Mr. Obama in a reasonably good position in Michigan. It's extremely doubtful that he's ahead by double-digits there - and almost certainly not by 14 points - but the forecast projects a six- or seven-point victory for Mr. Obama in Michigan on Nov. 6. Let me explain how it comes to that conclusion.

In the table below, I've broken down the Michigan polls into two categories: those conducted by firms that frequently survey states outside of Michigan, and those that poll Michigan only, or Michigan and one other state.

The national polling firms, like Rasmussen Reports, Public Policy Polling, and Marist College, show Mr. Obama ahead by an average of seven points in Michigan. By contrast, the Michigan-only pollsters show him ahead by only two points.

The advantage of the national pollsters is that we have a good sense for how to interpret their results. Some are Democratic-leaning and some are Republican-leaning - but whatever “house effect” they tend to show, it is usually fairly consistent from state to state.

By contrast, when a polling firm takes surveys in one state only, and its results diverge substantially from the consensus, it's harder to tell what is going on. It could be that there is something about Michigan in particular they are seeing. Or it could be something more intrinsic to the methodology: that if the firm shows good numbers for Mr. Romney in Michigan, for example, it would also do so in Colorado, or Virginia, or Ohio, if only they surveyed those states.

The model is designed to mostly prefer the latter interpretation. If a local polling firm diverges substantially from the consensus, it infers that the results show a strong “house effect,” rather than providing any particular insight about the state. It doesn't entirely dismiss the poll, but it treats it skeptically.

But isn't local knowledge a good thing? It can be, and there are some outstanding state polls conducted by local newspapers or local universities, which are rewarded accordingly in our pollster ratings . However, there are also some very questionable ones.

In Michigan, there are two local polling firms in particular that have shown poor numbers for Mr. Obama. One is Mitchell Research, which just published a poll showing a tie there. The other is Foster McCollum White Baydoun, which had Mr. Romney ahead by four points in its last survey.

Foster McCollum White Baydoun actually does survey one state apart from Michigan: it released a poll of Florida last week. In that survey, it showed Mr. Romney ahead by almost 15 points - perhaps the biggest outlier we've seen in any individual poll so far this year.

As I mentioned when that Florida poll came out, this firm anticipates a big decline in participation among groups like younger voters that are ordinarily inclined to support Democratic candidates. The initial news release of the Florida poll appeared to show that the firm anticipated that just 11 percent of Florida voters would be under age 50 - in contrast to about half the Florida electorate in 2008, according to exit polls. A principal of the firm, Eric Foster, later wrote me to say that the news release had been incomplete, and voters under the age of 50 had actually been weighted to make up 27 percent of the poll instead.

That's still an implausibly large drop-off, in my view, from 2008. It may be because the Foster McCollum White Baydoun likely-voter model looks for voters who, as they described it, participated in “odd year municipal and county elections” - where turnout is tiny as compared with what it will be on Nov. 6. In any event, this method seems likely to produce extremely Republican-leaning results - whether applied to Michigan, to Florida or to any other state.

At least Foster McCollum White Baydoun have a methodology of some kind, even if it isn't one that I would recommend. What Mitchell Research is doing in Michigan is a little more troubling.

The head of Mitchell Research, Steve Mitchell , wrote a long memo accompanying his poll release on Monday. In that poll, he weighted the survey to assume that African-Americans would make up only 8 percent of Michigan's turnout. By contrast, black voters represented 12 percent of the turnout in Michigan in 2008 according to exit polls, and 14 percent according to another source, the Current Population Survey. Blacks also made up 13 percent of Michigan's vote in 2004 and 11 percent in 2000, according to exit polls. African-American participation is sometimes lower in midterm election years, but blacks were 12 percent of Michigan's electorate in 2006, the exit poll reported that year. (There was no exit polling in Michigan in 2010.)

So why only 8 percent this year? Mr. Mitchell wrote that he simply doesn't believe the exit polls:

“African-American participation in this poll is 8%, not 12%, which is the percentage of the population but not likely voters. I do not believe blacks represented 12% of the vote in 2008 and I don't believe they will in 2012. Having polled this state for 26 years, blacks have represented about 7%-8% of all votes cast in every statewide race. At best, it went to 9% four years ago. It will not reach that level this year.”

What is the evidence for Mr. Mitchell's claim? He didn't present any of it in the memo. The exit polls certainly aren't perfect, but they've been consistent from year to year and also seem to agree with Census Bureau data. (Another fact that Mr. Mitchell cites - that African-Americans represent 12 percent of the overall population in Michigan - is also slightly incorrect; they made up 14 percent of Michigan's population as of 2011, according to the Census Bureau.)

Perhaps Mr. Mitchell means that only about 8 percent of respondents are black when he takes his surveys. But response rates to political surveys have been much lower among African-Americans for many years. The problems may be worse in a su rvey, like Mr. Mitchell's, that does not call cellphone voters, since African-Americans are more likely to rely on cellphones in lieu of landline phones.

It is precisely because of this problem that essentially all polling firms do weight by race, usually calibrating their numbers based on Census Bureau or exit poll data. Mr. Mitchell, however, seems to have his own ideas about what Michigan's electorate will look like. There is a certain amount of art in political polling, but I've never heard of a pollster treating the demographic makeup of a state as essentially a matter of opinion.

Mr. Mitchell's demographic weights account for much of the difference between his polls and those of the other firms that are active in Michigan; a decrease in the African-American vote from 12 percent to 8 percent would harm Mr. Obama's numbers by a net of about four percentage points there. If you have some reason to discount the Mitchell Research and Foster McCollum White Baydou n polls, it is much harder to make the case that the presidential and Senate races in Michigan are a “tossup,” as some other Web sites have characterized it.

To be clear, our forecast model doesn't “know” all of this detail about the Mitchell Research and Foster McCollum White Baydoun polls. It discounts the results from these firms simply because that's what it's programmed to do whenever it encounters a local polling firm whose results diverge substantially from the consensus; this method works to Mr. Obama's benefit in some states, and Mr. Romney's in others.

But my personal view is that the model is making the right inference in the case of Michigan, and that the methodological choices that these polls have made are hard to defend. Perhaps 90 or 95 percent of the time, taking a simple average of the polls will work just about as well as the more complicated FiveThirtyEight method. But this is rare instance where taking all the polls at face value may be a mistake, and the additional checks-and-balances the FiveThirtyEight method applies are worth the trouble.

Aug. 26: Little Momentum in Polls in Advance of Conventions

By NATE SILVER

The FiveThirtyEight forecast for Sunday gives Barack Obama a 69.4 percent chance of winning the Electoral College on Nov. 6. That's essentially unchanged from Saturday - although there have been some modest shifts back and forth in the numbers over the course of the past two weeks since Mitt Romney named Representative Paul D. Ryan of Wisconsin as his vice-presidential nominee.

Mr. Romney was given a 28.0 percent chance of winning the Electoral College on Aug. 10, the day before he officially announced Mr. Ryan as his pick. The forecast then moved somewhat toward Mr. Romney after a series of improved polling in swing states for the newly minted Republican ticket, achieving a peak of 33.3 percent on Wednesday. It has since receded slightly to 30.6 percent, however, as Mr. Obama held leads in a number of swing state polls late last week.

These shifts could be consistent with a small vice-presidential “bounce” for Mr. R omney which has since faded - perhaps as less favorable stories for Republicans, like the comments on abortion and rape made by Representative Todd Akin of Missouri, have come to dominate the news cycle.

But these are only very minor differences - the model estimates that Mr. Romney gained a net of perhaps one percentage point in the popular vote after his selection of Mr. Ryan, and has lost perhaps half a percentage point since then. Changes of that magnitude could potentially be caused by statistical noise, as well as by real shifts of opinion.

What's clearer is that Mr. Romney did not get as large a bounce in the polls from his vice-presidential pick as most past candidates have - a fact that can arguably be read as a bearish sign for him.

The Republicans do seem to have made some headway in Mr. Ryan's native state of Wisconsin, however. And their last few polls in Florida have been been reasonably strong, defying the conventional wisdom that suggested M r. Ryan's views on entitlement programs could be damaging among the senior population in that state.

Can Romney Achieve Focus in Tampa?

Political coverage over most of the past two months has veered between hard news events like the Supreme Court's ruling on Mr. Obama's health care bill, and a set of personal issues that can occupy the news cycle for days at a time, like Mr. Romney's tax returns or the comments made by Mr. Akin.

Both campaigns also seem to have been in trench-warfare mentality, with a focus on day-to-day tactics rather than long-term strategy.

The Republican convention in Tampa could represent an opportunity for Mr. Romney to break through the noise and show some more sustained improvement in his polling. Conventions almost always do produce bounces in the polls. These bounces fade, but an especially large or an especially small bounce can be an indicator of how the rest of the race is likely to proceed.

However, the Republicans face an unusual pair of challenges at their convention: first, that the interval separating the Republican convention from the Democratic one in Charlotte is extremely short; and second, that the news media's attention could be divided between the developments in Tampa and those in the Gulf of Mexico, where Hurricane Isaac is a threat to New Orleans and other cities on the Gulf Coast.

Sunday's Polls

The only new state poll out on Sunday was in Ohio, where the Columbus Dispatch published a poll showing Mr. Obama and Mr. Romney tied at 45 percent each.

The Columbus Dispatch poll is unusual: It is conducted entirely by mail, with surveys sent out to a random selection of Ohio voters.

The potential problem with this polling method is that because returning a mail survey requires more effort than taking a phone call, the response rates are likely to be lower and could potentially overrate the strength of the candidate with the more enthusiastic supporters .

However, it might be noted that all forms of survey research - certainly including telephone polling - face a problem of low response rates. And historically, the performance of the Columbus Dispatch poll has been about average, with some strong years and some poor ones.

If its survey differed greatly from the consensus, I would be a little wary of it - but showing a tie in Ohio is not that different from the average of surveys there, which puts Mr. Obama ahead but by a narrow margin. In short, I don't have any problem including Dispatch's survey alongside the rest of the Ohio polls. Mr. Romney's chances of winning Ohio improved slightly in our forecast as a result of the poll, increasing to 33 percent from 31 percent on Saturday.

Mr. Romney got slightly unfavorable results in the national polls that were out on Sunday, however, with the Rasmussen Reports tracking poll showing Mr. Obama pulling into a 2-point lead among likely voters - a better result tha n it appears for Mr. Obama since Rasmussen's polls have been somewhat Republican-leaning this cycle.

There was also a poll of likely voters from the Tarrance Group, a well-respected Republican polling firm, which gave Mr. Obama a 1-point lead among likely voters.

How Will Party Conventions Affect the Presidential Race?

By JOHN SIDES

The Republican and Democratic national conventions bring the possibility of change to this static presidential race.  But how will it change, and how much?  Here is some of what we know.

The party conventions typically cause the largest swings in the polls during the presidential campaign.  During the convention season, the polls are unstable and likely to change more than at any point afterward, according to research by the political scientists Robert Erikson and Christopher Wlezien.

As is well-known, a party convention typically creates a “bump” or “bounce” for the candidate being nominated. Across presidential elections from 1964 to 2008, this has typically meant a 5-6 point increase, according to both political science research and the tabulations of pollsters like Gallup.

This leads to a first question: Why a bounce at all?  Why would the conventions have this effect in the first place?  After all, they are largely symbolic events, lacking in real drama and thus covered less and less by news organizations and watched by fewer and fewer viewers.  (This paper by political scientist Costas Panagopolous documents the decline.)

One reason is that conventions generate a larger “dose” of information than is provided by the daily ebb and flow of news coverage.  Thomas Holbrook's study of campaigns from 1984-92 found that front-page coverage of each presidential candidate rose sharply during his party's convention.

A second reason is that the news coverage during the convention favors the candidate being nominated. There are typically no increases in the coverage of the Republican candidate during the Democratic National Convention, and vice versa. Thus, unlike the later debates, a convention allows the candidate to present himself, Mr. Holbrook writes, “in a relatively uncontested format.”  So conventions typically generate not just news, but favorable news for the candidates.

Next question: How big is the bounce?  The bounces have ranged from essentially zero (John Kerry in 2004) to about 14 points (Barry Goldwater in 1964 and Bill Clinton in 1992).  Two things appear to affect the size of the bounce, according to Mr. Holbrook in a recent blog post.  One is how well the candidate is doing, relative to expectations. Here, we can think about expectations in terms of the underlying political and economic fundamentals.  So a candidate running behind what the economy and other fundamentals would predict should get a larger convention bounce.  A second factor is the timing of the conventions.  Traditionally, the party whose convention occurs first - the party currently not in control of the White House - has had the larger bounce, which makes sense if the challenger is not as well-known as the incumbent and therefore voters have not formed as strong an opinion about the challenger.  However, holding the conventions close together and somewhat later in the campaign season, as in 2008 and 2012, seems to mute the advantage of going first.

Final question: Will the bounce persist? Will the changes during this period have lasting benefits for President Obama or Mitt Romney?  The term “bounce” makes it seem as if the candidates numbers will go up and then down, rendering any impact temporary.  In fact, the conventions tend to leave a more permanent imprint.  Mr. Erikson and Mr. Wlezien in their study of the 1952-2008 presidential elections find:

On average, the party that gains from before to after the conventions maintains its gains in the final week's polls…Although the convention season is the time for multiple bounces in the polls, one party ends up with an advantage when the dust clears.

Of course, this is an average result, not a description of every election.  In 2008, Mr. Obama was behind in several polls after the Republican conve ntion, only to cruise to a comfortable victory.

What does this mean for 2012?  I'll leave the exact forecast to Nate, but I would expect only small bumps for either party.  Neither candidate is really polling above or below expectations at the moment.  The fundamentals going into the race suggest an Obama victory, but by a relatively small margin - and that's exactly where the polls are right now.  Moreover, scheduling the convention late in the summer and back-to-back should mitigate their impact.  And there is only a small number of self-described undecided voters, which may help explain why another high-profile event, the naming of Representative Paul D. Ryan as the running mate, has not really moved the national polls.

To be sure, even “small bumps” might be enough to put the race on its head, moving Mr. Romney from a slight underdog to a slight front-runner.  If that proves true, the question is whether Mr. Romney will sustain that lead through Ele ction Day.

Aug. 25: An Above-Average \'Likely Voter Gap\' for Romney

By NATE SILVER

CNN's latest national poll, released on Friday, contained a mix of good, bad and indifferent news for each candidate.

The good news for Barack Obama? Among registered voters, he led Mitt Romney by nine percentage points, with 52 percent of the vote to Mr. Romney's 43 percent.

However, Mr. Obama led by just two percentage points, 49 to 47, when CNN applied its likely voter screen to the survey. This is the first time this year that CNN has reported likely voter results.

Holding a two-point lead among likely voters is not an especially bad (or good) number for Mr. Obama, since it is highly consistent with the way that our forecast sees the overall race right now.

What's worrisome for him, rather, is the large gap in the poll - seven points - between the likely voter and registered voter results.

In our forecast, we use cases like these, in which pollsters report both sets of results, to help calibrate our likely voter adjustment. It is typical for likely voter polls to show more favorable results for Republicans than registered voter ones, and so our adjustment shifts registered voter polls toward Mr. Romney before incorporating them into our averages.

But in past presidential elections, the difference has been relatively small - favoring the Republican candidate by about 1.5 percentage points, on average. (The gap can vary more in midterm election years, when turnout is lower.)

This year, however, most of the polls to report both sets of results have shown a larger gap than that. The seven-point difference that CNN had is not typical. But we've been tracking these results whenever state and national polls report them, and the median difference between registered voter and likely voter results has been about three points so far in Mr. Romney's favor - about twice as large as the historical average.

As a technical matter, our forecast hedges against these po lls somewhat. Not that many polling firms have reported these numbers so far, with most listing either registered voter or likely voter results but not both. And likely voter models can be a bit erratic when applied this early in the election cycle, when a lot of voters are not yet tuned in. It estimates that the gap will wind up being in the neighborhood of two-and-a-half points instead.

All this may seem like splitting hairs, but in a close election like this one, it can matter. A turnout gap is one of the bigger reasons for Democrats to be concerned about the election. It looks - for now at least - that Mr. Obama has enough support among the broader universe of American adults to win another term. But he could easily lose his office if those Americans who might be inclined to support him are not registered, or do not turn out to vote.

And yet, while this is good news for Mr. Romney, it does not qualify as great news for him. The reason is that, although a numb er of national polls are still reporting registered voter results, almost all the state polls released over the last month or two were conducted on a likely voter basis - his turnout advantage should already be priced into them. But Mr. Obama still leads more often than not when likely voter polls from battleground states have been released.

It's all a bit of a mess, frankly. I suspect that part of the problem is that polling firms are applying likely voter methods that might have been designed 30 years ago to a modern polling universe of extremely low response rates (even the most thorough polling firms can only get about 10 percent of voters to return their calls), cellphone-only households, and an increasingly diverse and partisan electorate - and that is producing erratic and unpredictable results. There's always some uncertainty about just who will turn out to vote, but there is more of it than usual this year.

Little Traction for Romney in Pennsylvania

< p>One state where there seems to be less doubt about the outcome is Pennsylvania, where a Philadelphia Inquirer poll on Saturday was the latest to show Mr. Obama with a solid lead, putting him ahead 51 to 42 among likely voters. Mr. Obama has led in 21 consecutive polls of Pennsylvania and his margins seem to have widened a bit recently; the forecast model now gives him an 89 percent chance of winning it on Nov. 6.

Mr. Romney and affiliated groups have not been spending very much money in Pennsylvania, a decision that I have argued before was conceding the state too easily to Mr. Obama.

But at this point, Mr. Romney almost might as well follow through on that strategy. It will be harder for him to make up ground in Pennsylvania now - whereas Wisconsin opens up some additional Electoral College possibilities for Republicans, given how the polls have tightened there since Mr. Romney's selection of Representative Paul D. Ryan as his running mate.

Pennsylvania has twice as many electoral votes as Wisconsin - but even accounting for that, it has fallen into eighth place on our list of tipping point states, whereas Wisconsin has risen to third.

A Pollster Finally Gets Missouri Right?

Rasmussen Reports had a flashy number for Mr. Obama in the Missouri poll it released on Friday - showing Mr. Obama ahead by a point there. But this was counterbalanced by a Mason-Dixon poll for the St. Louis Post-Dispatch, which had Mitt Romney up by seven points instead. Mr. Romney had led by nine points in their previous survey of Missouri.

The Mason-Dixon poll also had the Democratic incumbent, Claire McCaskill, pulling into a lead in the Senate race in Missouri, however. She now leads by nine points, a substantial swing from their July poll, when Representative Todd Akin led by five points, before his much-criticized remarks about rape.

I know it cuts somewhat against the FiveThirtyEight spirit to lump polls into binary categ ories of “right” and “wrong.” But the Mason-Dixon poll seems like a far more sensible take on the state of play in Missouri than the others we've seen since Mr. Akin's remarks - showing a significant change in the Senate race there but a marginal impact at best on the presidential contest.

Romney's Momentum Fades Slightly Heading Into Conventions

Over all, with a set of reasonably favorable state polls on Thursday through Saturday after some poor ones earlier last week, Mr. Obama has rebounded a bit in our Electoral College forecast. The model now gives him a 69.3 percent chance of winning it, up from 66.7 percent on Wednesday. There's also been a tiny shift back toward Mr. Obama in the national tracking polls, with Gallup now showing the race tied, and Rasmussen Reports putting Mr. Obama one point ahead as of Saturday.

It's hard to tell whether there was a vice presidential ‘bounce' for Mr. Romney that has since reversed itself - or whether this is all just a bunch of statistical noise. In this case, it may be something of a moot point, since the party conventions are likely to reset the momentum in one direction or another anyway.