Interestingly, it looks like Specter foreshadowed his move yesterday. See this exchange with Neal Conan on NPR's "Talk of the Nation," summarized on the show's blog today: 

[I]n the course of the conversation, Neal read an email from Russ:

Many suspect strongly that Republicans are going to dump Senator Specter in the coming midterm elections. Many Pennsylvania Democrats would support him as an independent, myself included, but we are watching his record carefully.

Neal continued, "Well, that election is, I guess, more than a year away. Senator Specter, nevertheless, in opinion polls currently, you trail a Republican opponent."

In retrospect, the Senator's response is very illuminating:

Well, it is true that the polls are bleak. When I voted for the stimulus package, one of just three, and was in position, along with Senator Snowe and Senator Collins, to provide the decisive votes, there was a very strong adverse reaction. There was a resolution filed in state committee to censure me. The state chairman and the national chairman said they didn't know if they could support me. My office was picketed. And it's a tough proposition. I've overcome some challenges before, and I'm working on a game plan. [E/A.]

For what it's worth, I'm not quite as hard on Specter as Chait is. Yes, this was clearly an opportunisitc move designed to save his Senate seat. On the other hand, as Specter points out in his announcement, the facts on the ground have really changed. Pennsylvania has become a much more Democratic-leaning state in recent years, and I don't begrudge a politician for bowing to reality. Certainly politicians have moved further along the partisan and ideological spectrum for less noble reasons. 

--Noam Scheiber