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Pennsylvania Revisited

I’ve been fielding a lot of questions about the Public Policy Polling survey in Pennsylvania’s Senate race that gives Representative Joe Sestak, the Democrat, a 1-point lead over the Republican Pat Toomey.

I wrote last week that a comeback by Mr. Sestak, who had been trailing Mr. Toomey by around 6 or 7 points in most surveys until now, was not more likely than the chances assigned to him by our model, which had been about 5 percent.

Obviously, we have new information in the form of the Public Policy Polling survey. But a few things to keep in mind:

  • Usually, general elections don’t turn on a dime without good reason — and going from 7 points down (or 9 points, as Public Policy Polling had Mr. Sestak in its August survey) to 1 point ahead would be fairly unusual. It can happen, but it doesn’t happen that often.
  • You’ll rarely make a mistake by holding out for more data. While pollsters have been distracted by states like Delaware that are unlikely to be close on Election Day, Pennsylvania has received uncharacteristically scant polling. So far, no other nonpartisan poll confirms Mr. Sestak’s surge, and the next-most-recent survey of the state, from Rasmussen Reports, gave Mr. Toomey a 10-point lead — the largest advantage Rasmussen had given him to date. One poll does not a trend make, and even when several polls do agree on a trend, it can often reverse itself.
  • Public Policy Polling conducts surveys for Democratic candidates (and Daily Kos, a liberal blog) in addition to issuing surveys under its own name. Until recently, we have not found an especially large “house effect” for Public Policy Polling — that is, they’ve had plenty of surveys showing poor numbers for Democrats. But lately, such an effect has arguably become more noticeable:  they are the only pollster, for instance, to show the Democrat Michael Bennet with a lead in Colorado, although several other pollsters have shown that race tightening. And their survey of Arizona’s Third Congressional District, which showed the Republican Ben Quayle trailing in a district that ordinarily leans  Republican, has raised a few eyebrows, although Mr. Quayle may be an unappealing enough candidate that the result is not necessarily implausible.
  • The most prudent conclusion is that Mr. Toomey still holds a lead, but it is probably smaller than the one he held before. Still, even small leads can be meaningful at this time of year, and Mr. Sestak may not be helped by the fact that Democrats are performing poorly in the governor’s race in Pennsylvania as well as several competitive House races around the state.

    When I reran our numbers with the Public Policy Polling survey included, Mr. Sestak’s chances were improved, but only to 10 percent.

    With that said, our Senate model does not use partisan polling in its forecasting. But publications like The Hotline have indicated, and some contacts of mine have related, that not only do Democratic internal polls show the race tightening, but Republican ones do as well. For that reason, I would be inclined to take Mr. Sestak’s side of the 9:1 odds that our model is offering him.

    We’ll need quite a bit more evidence, however, before the race can rightly be thought of as a tossup.

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