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Sept. 2: Split Verdict in Polls on Romney Convention Bounce

My view is that the consensus of evidence so far points toward Mitt Romney having received a small bounce in the polls of perhaps two or three percentage points from the Republican convention in Tampa, Fla.

Oddly enough, however, you can’t really find a poll that seems to reflect a 2- or 3-point bounce exactly. Instead, there have been some polls where Mr. Romney’s bounce has been a bit larger than that, and others where there is little sign of a bounce at all.

On the favorable side for Mr. Romney is the Rasmussen Reports tracking poll, which now shows him four percentage points ahead of President Obama. That represents a 6-point swing toward Mr. Romney compared with the poll Rasmussen Reports released immediately before the convention, although the bounce is smaller (about three percentage points) measured relative to the long-term average of their surveys.

An online national tracking poll from Ipsos also shows a bounce for Mr. Romney, although its magnitude has been shifting a bit from day to day. Based on the latest iteration of the poll, the race is now tied among likely voters at 45 percent each, reflecting a 4-point swing toward Mr. Romney from Ipsos’s poll before the convention. And the shift has been larger — a net of seven percentage points toward Mr. Romney — in the version of the poll that tracks the preferences of registered voters instead.

But there are also two clearly unfavorable data points for Mr. Romney. One is the Gallup national tracking poll, which still shows Mr. Obama with a 1-point lead — actually a bit worse for Mr. Romney than Gallup’s last survey before the convention, when he had led by one point.

Gallup’s survey still contains a fair number of interviews that were conducted before the convention began, and there is a bit of a silver lining for Mr. Romney in that Mr. Obama’s approval rating ticked down in the poll on Sunday. Since Gallup’s approval ratings are based on a 3-day polling average while their horse race numbers are based on a 7-day average, that could indicate Mr. Romney may still gain a bit of ground in the poll once it fully rolls over to post-convention interviews.

On the other hand, Gallup’s Frank Newport suggested in a blog post on Sunday night that polling asking voters for their views of Mr. Romney’s speech had produced evaluations “at the very low end of the scale” as compared with previous acceptance addresses. In any event, if Mr. Romney’s bounce had been more emphatic, it would be a little easier to find right now.

The other bad numbers for Mr. Romney are in a pair of surveys that Public Policy Polling released Sunday night. In those surveys, Mr. Obama held a 1-point lead in Florida, while the two candidates were exactly tied in North Carolina.

Those numbers are broadly consistent with how Public Policy Polling had been showing the races in those states previously, although you can argue that there has been a shift of a point or so toward Mr. Romney depending on what baseline you choose. And it is always worth remembering that Public Policy Polling’s surveys have been a bit Democratic-leaning this cycle — by about one or two percentage points relative to the consensus.

Still, I do not think we should be interpreting these Public Policy Polling surveys as anything other than poor data points for Mr. Romney. A Republican candidate who was essentially tied in slightly red-leaning states like North Carolina and Florida in polls conducted immediately after his convention would be in some trouble.

Fortunately for Mr. Romney, another North Carolina survey out on Sunday night, from Elon University for the Charlotte Observer, had somewhat better numbers for him there, giving him a 4-point lead among likely voters. This is their first survey of the state this year so there are no trendlines for comparison, but since our forecast model had been showing Mr. Romney as a 1- or 2-point favorite in North Carolina before the conventions, this result seems fairly consistent with a 2- or 3-point bounce for him.

It is important to remember that there is quite a bit of noise in the polls based on statistical variance, along with the methodological choices that different polling companies make. It would be nice if every poll showed a nice, tidy bounce of about the same magnitude for Mr. Romney, but rarely are the polls so well behaved.

Our forecast model builds in an adjustment for the party conventions; it treats anything larger than a 4-point bounce as being a favorable sign for Mr. Romney, and anything smaller than that as being an unfavorable one.

This could change as we get more data, but for the time being it looks like Mr. Romney’s bounce will be a bit shy of that 4-point threshold. Thus, the forecast has moved toward Mr. Obama over the past few days; it now gives him a 74.5 percent chance of winning the Electoral College, his highest figure to date.

Our “now-cast”, however, which does not build in an adjustment for the conventions, moved slightly toward Mr. Romney on Sunday. Its estimate is that he would have a 29 percent chance of winning the Electoral College in an election held today, up from 27.7 percent on Saturday.

In other words, the polling over the past few days would look slightly favorable for Mr. Romney under ordinarily circumstances, but it reads somewhat bearishly for him given that candidates typically have modestly inflated numbers after their party conventions.

The real action, however, will be after the Democratic convention in Charlotte, N.C., when the expectation built into the forecast is that Mr. Obama will post numbers that are a point or two better than his long-term averages.

Something in the neighborhood of a 4-point lead for Mr. Obama in national polls and polls of key states conducted just after Charlotte would be in line with these expectations. If Mr. Obama has an advantage in the mid-to-high single digits instead, that will start to look problematic for Mr. Romney, whereas, if the race is roughly tied or Mr. Obama leads by only a point or so, Mr. Romney may credibly claim to have gotten the better of the conventions despite having a relatively small bounce himself.

But the one eventuality we probably can take off the table is the notion that Mr. Romney would emerge from his convention with unmistakable momentum, as Ronald Reagan did in 1980 or Bill Clinton did in 1992. His bounce may turn out to be “just fine” once we see a few more polls, and how the numbers move after Charlotte. But Mr. Obama is unlikely to make it easy for Mr. Romney.

Nate Silver founded and was the editor in chief of FiveThirtyEight.