The American Spectator : The Spectacle Blog : Specter's Choice

Clearly, Arlen Specter was facing a tough primary fight against Pat Toomey and from his statement it's pretty clear he determined that he couldn't win as a Republican. But it's no guarantee that he'll be able to win the Democratic nomination so easily either, especially given his vow not to reverse his position on card check yet again. If Specter had made this party switch right after his vote in favor of the stimulus package, and before he decided to oppose card check, he would have been in a far better position to claim the Democratic nomination. Now, as Kos writes:

Interestingly, he remains a foe of EFCA, which means that labor is free to fund and help a real Democrat in the Democratic primary. Bizarre choice. Had he decided to back EFCA, as he has always done so in the past, he'd have labor's full support. Now, he gives the opposition an opening to take him out in the Democratic primary.

This is a huge blow for Republicans hoping to stop Obama's agenda in the Senate. Specter had been moving to the right on issues such as card check because he was concerned about the challenge from Toomey, but if he's facing a tough battle against a liberal opponent in the Democratic primary, the opposite dynamic comes into play and he's likely to move even further to the left. The only way he'll get the Democratic nod is if he reliably votes with the administration.