Why Specter switched

Ignore any high-minded language. Here's the real reason:

The numbers already don't look good for Sen. Arlen Specter.

In a Rasmussen Reports poll out this morning [April 24], the Pennsylvania Republican lags behind GOP primary challenger Pat Toomey by more than 20%. Just 30% of Republican voters in the Keystone State say they would vote for the incumbent, while 51% are throwing their support behind Toomey.

It's all about self-preservation.

Update: Here is Specter just last month:

Sen. Arlen Specter (R-PA) told The Hill that he will not become a Democrat -- but he might become an independent.

"I'm staying a Republican because I think I have a more important role to play there," he said. "I think the United States very desperately needs a two-party system. ... And I'm afraid that we're becoming a one-party system, with Republicans becoming just a regional party."

The only thing that changed is the polling data.

Update: Lefty Jonathan Chait calls Specter an "unprincipled hack":

When a politician switches parties, it’s customary for the party he’s abandoned to denounce him as an unprincipled hack, and the party he’s joined to praise him as a brave convert who’s genuinely seen the light. But I think it’s pretty clear that Specter is an unprincipled hack. If his best odds of keeping his Senate seat lay in joining the Communist party, he’d probably do that.

To be sure, Specter is a real moderate on some issues, but his contortions are so comical that no principled read on his actions is very plausible. Specter favored the Employee Free Choice Act favored by labor, turned against it when he faced a primary challenge, and then abandoned his party altogether when it became clear he couldn’t win his primary. In the meantime, he came out in favor of a Hooverite spending freeze after backing the stimulus bill.

Update: Chait isn't alone on the left in his disdain for Specter.

Update: Interesting nugget I found out about Specter last week -- he was a registered Democrat until his mid-30s. Indeed, he was even the defense attorney for convicted killer and radical leftist Ira Einhorn.