Here's Karl Rove describing his reaction to the leap:
Actually Karl, it turns out that Specter wasn't a "life-long Republican" as you claimed; he was a Democrat until 1966 when he joined the GOP, and he simply realized that during the last four decades (and especially the last one) the party has moved too far to the right for the moderate senator.
I still don't think I can forgive Specter for his awful interrogation of sexual-harassment victim Anita Hill during the Clarence Thomas confirmation hearings; not unless he publicly apologizes to Ms. Hill. If after that she forgives him, then I'll forgive him.
But for now, I have to agree that it's a shrewd political move by Specter, which will ensure his spot on the ballot next year, as he would have faced a strong and very conservative GOP challenger in a primary for the state's remaining 21% Republican vote.