-He left the Republican Party because he's too liberal and couldn't survive a Club for Growth-backed primary challenger.
-He may be denied the Democratic nomination because he doesn't support card check legislation, making him anathema to labor unions.
So he's too liberal for Republicans and too conservative for Democrats. If he gets denied the Democratic nomination, that strikes me as a win all around. The GOP is happy that he pays a big price for leaving the party, the Dems are happy because they can get a real liberal elected to the seat, and political scientists are happy because we've got some great ethnographic evidence about polarization. The only real loser here would be Specter, and I can live with that.
Luckily for him, though, Specter has always been ideologically flexible enough to win office under pretty much any circumstances, and he'll probably figure something out here. (He did come up with the single bullet theory, after all.) Either he'll change his mind on card check, some other senator will pull the Employee Free Choice Act off the Senate calendar for the year, or he'll pull a Lieberman and run under the "Pennsylvanians for Specter" party.