Specter

Unlike Nate Silver, I think Specter's move is a biggish deal, because he will have to pander to Democratic interests to avoid a primary challenge from the left. Specter's potentially pretty vulnerable in a Democratic primary, as Silver says:

Specter can't be too cute about this, or he might have primary problems on the left. The Republican nominee is probably going to be Toomey, who will be an underdog against any sentient Democrat. Why should the Democrats settle for a Liberdem when they can probably get Pennsylvanians to elect a mainline Democrat along the lines of Bob Casey?

This being so, it seems like (a few random "principled" stands aside) Specter will have to become substantially less conservative, at least over the next few months, than he has been -- probably less so than Ben Nelson, Mark Pryor, and others who are tolerated only because they represent very red states. This has two interesting implications:

1. It's not true that "the Senate's fortunes will still be determined by a group of about a dozen moderate senators from both parties" -- they will be determined by a group of moderate Democrats. The implicit justification for the Blue Dogs' "bipartisan" meddling was that they needed to make compromises to get the moderate Republicans on board. Specter's move removes this justification; henceforth the Blue Dogs will have to assume the responsibility for holding things up.
2. The moderate Republicans are almost nonexistent. Mike DeWine and Lincoln Chafee lost in 2006; John Warner and Chuck Hagel retired in 2008; Ted Stevens, Gordon Smith, and Norm Coleman lost in 2008; and now Specter's a Democrat. Most of these were consistently to the right of Olympia Snowe and Susan Collins, and consistently to the left of all other Republicans, so there's now a large ideological gap between SnoweCollins and the rest of the Republican Party. I'm curious to find out how this situation develops.