Then why, regardless of which side you ultimately caucus with, aren't you running as an independent, Senator?
Here's the thing:
It has become clear to me that the stimulus vote caused a schism which makes our differences irreconcilable.
It might be a pretext, but if Specter is leaving primarily because of the stimulus vote - and not the GOP's social conservatism, foreign adventuring, anti-intellectualism, etc. - then I think it was time for him to go. That may sound rash given the GOP's heavily reduced position in the Senate. But opposition to the stimulus represents a legitimate focal point of opposition centered on fiscal conservatism and limited government that should be able to appeal to a wider range of individuals than the Bush-era GOP. It's a better least common denominator around which to form a revitalized party, as I think Rep. Paul Ryan, among others, has shown.
If the stimulus was the decisive issue for Specter - which should have been the easiest, least controversial part of the GOP agenda to agree with - then I'm not surprised he left. It's unfortunate, though, as it represents a political "tragedy of the commons" wave overpowering the reality of scarcity and the prospect of unsustainable deficits.
Everyone wants a stimulus, and, wrapped up in the numbers of the Obama surge, Specter caved to save his seat.
Had he decided to run as an independent, no matter how much more difficult that would make his re-election, my assessment would have been a bit less harsh - that would look far more like a move grounded in true loyalty to the citizens of Pennsylvania and the notion of public service in the face of looming public challenges.
UPDATE: Specter on why Specter should not have done what Specter just did.