I don’t think we should take the fact that Specter has sometimes voted in a moderately progressive manner to mean that he’s “really” a moderate progressive. Nor do I think we should take the fact that he has sometimes voted in a hard-right manner to mean that he’s “really” a solid conservative. He’s just flexible. Look at, say, anything he said today and compare it to this ad (hat tip to Chris Bowers) he made when trying to convince Pennsylvania Republicans to back him in the 2004 primary against Pat Toomey:
I think the evidence suggests that a party switch will cause Specter to drift ideologically somewhat to the left, but clearly this is a guy capable of adopting a diverse set of views depending on circumstances.