The New Republic's Jonathan Chait isn't exactly welcoming Arlen Specter on board:
I think it’s pretty clear that Specter is an unprincipled hack. If his best odds of keeping his Senate seat lay in joining the Communist party, he’d probably do that.
To be sure, Specter is a real moderate on some issues, but his contortions are so comical that no principled read on his actions is very plausible. Specter favored the Employee Free Choice Act favored by labor, turned against it when he faced a primary challenge, and then abandoned his party altogether when it became clear he couldn’t win his primary. In the meantime, he came out in favor of a Hooverite spending freeze after backing the stimulus bill.
I wonder if the Democrats will be able to wrangle him any better than the Republicans could. Doubtful, for the same reason no one from either party can nail Jell-O shots to the wall.
Meanwhile, Andrew Sullivan calls Specter "my kind of Republican in many ways."