Was it for sheer political calculation? He knew he could only win the next election as a Democrat? Undoubtedly this is a major fact in his decision, let's be honest about that. But it seems that the Republican party has gone so far right that he knew not only that he could not win reelection but that there was no place for him in the party.
Moderate Republicans are being punished and pushed out, that's the sad truth about today's GOP:
But here is the question for Democrats? Does this mean Specter will be more inclined to vote for Obama's agenda? I think it will and for two reasons: 1) he will not longer be pressured by Conservatives to vote against such measures, and 2) his reelection will depend on support from his new party. Specter is, obviously, something of an opportunist and will not want to bite the new hand that feeds him.
This is good news for key items like Health care reform and the transformation to Green energy.