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Specter Remains Vulnerable in Primary

Can you trust that new Rasmussen poll that shows Joe Sestak closing to within just two points of Arlen Specter in the Democratic senatorial primary in Pennsylvania?

I don’t know. Rasmussen polling released just in advance of election day has been quite accurate in the past. But their take on the race is inconsistent with what other polls have had to say — including other polls of ‘likely voters’ — a phenomenon that has become increasingly common with Rasmussen’s polling this year. The poll also has a small sample size, with 435 likely voters.

But suppose you ignore the Rasmussen poll and look at the Quinnipiac poll instead. That one, released last week, shows Arlen Specter ahead 53-32.

If this were a general election, the race would be almost over. Incumbent candidates very rarely lose once they’ve reached 50 percent in the polling. But instead, this is a primary, and the dynamics of primary elections are completely different. Most notably, in a primary, the ideological gap between the candidates is much smaller, and the ideological range among the voters is much smaller. In other words: most everyone’s a swing voter, and it doesn’t take that much to flip their preferences from one candidate to another.

Quinnipiac’s poll actually gets at his a little bit. They ask Specter and Sestak voters whether their mind is made up — or whether they could still change it. (More pollsters should ask this type of question more often.) The poll shows that only 58 percent of Specter voters, and 67 percent of Sestak voters, are firmly decided. So another way to look at the electorate is like this:

By this reckoning, about half (49 percent) of the votes are still in play: Sestak would need to win about 60 percent of them to get to 50 percent. That hardly seems impossible, nor even particularly difficult, especially given that Sestak has largely conserved his resources in the campaign. (It would be silly to conclude that such a strategy is ill-advised: even if it worked perfectly, we wouldn’t know it yet!)

Mind you, I don’t think Sestak is a perfect primary candidate. His voting record is not especially liberal. He ran a good campaign in 2006 to defeat incumbent Curt Weldon, but essentially all districts like Sestak’s, which has a PVI of D+3, were going to the Democrats that year. Meanwhile, Specter has become a reliable liberal vote. Although liberals (and others) might not trust him, a campaign against Specter might not produce the energy among the base, and especially within the netroots, that one against a candidate like Blanche Lincoln might. But Rasmussen poll or no, this race remains competitive.

Nate Silver is the founder and editor in chief of FiveThirtyEight.