http://www.washingtonpost.com/wp-dyn/content/article/2010/05/16/AR2010051602949.html?hpid=opinionsbox1

Is there really a debate about whether Kagan will be confirmed? The opposition will try to draw it out as long as possible, but there doesn’t seem to be any reason to think that they would actually filibuster – they admitted as much today. I prognosticate confirmation with 56 votes in favor. No GOP votes. Dem’s lose a few from moderate states, including, of course, Ben Nelson.

http://www.politico.com/news/stories/0510/37308.html

According to Michael McDonald, there are currently 4,311,206 registered Democrats (as of May) in Pennsylvania. This is down 3.8% from 2008. The Republicans now number 3,119,250 – also down 3.8% from 2008. So McDonald’s figures suggest there’s been no overall movement toward either party since 2008. (Interestingly, “unaffiliated” voters now total 1,013,903 which is down about 1.9% in that same period.) Keep in mind that unaffiliated voters cannot vote in the Pennsylvania Senate primary.

Now, one might argue as you do that Specter’s “base”, however defined, shifted Democrat in 2008. If I get a chance, I’ll try to break down his partisan support in 2004, when he last ran for election although I’m still not sure it will fully get at your question. But right now I don’t have evidence to fully assess your assertion.

More broadly, however, I don’t think we are disagreeing on why he switched parties: he wasn’t likely to beat Toomey in the 2010 Republican primary. Indeed, he barely beat him in 2004.

I’ve only begun looking at Pennsylvania polling data, so this is less precise than I’d like, but my initial read is that the polling data supports my anti-incumbent hypothesis. In a Specter-Toomey matchup, Toomey wins in part because of the anti-incumbent sentiment. In a Toomey-Sestak matchup that lacks a statewide incumbent, we see the normal political landscape, in which Pennsylvania Democrats outnumber Republicans, reassert itself, so Sestak runs closer to Toomey. Keep in mind that many general election likely voters – particularly independents – still have little familiarity with either Toomey or Sestak beyond a vague name recognition. So head-to-head matchups between those two are much more likely to break down according to general partisan lines than is a matchup with the better know Specter.

If I can, I’ll try to get data on party registration in Pennsylvania dating back to 2004 and revisit your hypothesis.

Can you elaborate on this…

“The irony is that Specter’s move left did nothing to dissuade Sestak from opposing him, and yet has likely made it more difficult for Specter to win the general election; even if he beats Sestak in the primary, polls indicate he will lose the general election to the Republican Pat Toomey.”

…in light of the fact that Sestak runs closer to Toomey than Specter does?

In my universe, the explanation is that Specter went Democratic because his base did — moderate GOPers and GOP-leaning independents swung into the Dem column in November 2008, and he was much, much less likely to win a GOP primary as a result. But he’s having trouble putting together a similar coalition in the Dem primary…partly because of Sestak’s brutally effective ad tying Specter to then-president Bush.

Thoughts?