In general, when a politician leaves a party, you ask "why?" With Specter, the more rewarding question is "who?" In this case, there's an answer: Pat Toomey and the Club for Growth. Toomey is the conservative ex-congressmen mounting a primary challenge against Specter. The Club for Growth is the monomaniacally anti-tax organization that's funding it. And Specter's defection is the direct result of that effort. He said as much at his press conference. That is to say that Specter's defection is the direct result of the Republican tendency to challenge unreliable politicians. It's a strategy that Democrats have, at times, envied: As the argument goes, Republicans have more party unity because they have less tolerance for betrayal. And that's probably true. There were a lot more Democrats who voted for Bush's tax cuts than there were Republicans who voted for Obama's stimulus bill.
But in recent years, no Senate Democrats have switched parties, while at least two Republicans have done so, and many more (including John McCain) have reportedly thought about it. Democrats, conversely, didn't even lose Joe Lieberman. It's an approach that makes for less unity and more votes. That frustrates Democrats when they see Evan Bayh and Ben Nelson freelancing their way across the stage. But it tends to look better when a Jim Jeffords gives Democrats the majority or Arlen Specter gives them their 60th vote.