I asked that exact question last month, when there were the first rumblings that Specter might switch parties. I still think it's a good question. I understand why Specter would switch parties: as Eric Kleefeld ably explains, he didn't have a choice if he wanted to win reelection in 2010: he wasn't going to win the GOP primary and running as an independent raises all sorts of structural impediments. But what's in it for Democrats? The very same political shifts in the Pennyslvania electorate that drove Specter from the GOP would have seemingly given someone like Joe Sestak a big leg up in a race against Pat Toomey. And Sestak would seem to be a much more reliable Democratic vote than Specter. After all, in announcing his party switch, Specter emphasized that he still opposed EFCA, so Democrats didn't get him to compromise there. Maybe, now that he's a Democrat, Specter will back off his threat to filibuster Dawn Johnsen's nomination. That's certainly the least he can do. But it'll be interesting to watch what happens in the Pennsylvania Democratic primary. Is it possible Sestak would go ahead and challenge Specter?