In case anyone out there still believed that these guys (& gals) made decisions based on principle, Specter's decision to switch parties should open your eyes. As Karl Rove just got done saying on FNC, this is pure political self interest.
Or, as my brother put it, "I suppose we should always assume self-interest of all politicians, that way these moves never surprise us." He--Specter--saw the writing on the wall with regards to Pat Toomey and made the logical, self-interested decision--he went Democrat.
He did what he had to do to remain in office.
Lots of people are saying that this is evidence that there is no room in the Republican party for people with politics like Specter's. If by "politics like Specter's" you really mean Republican In Name Only, then, the Democrat party is a friendlier place for guys like him--"friendlier" in the sense that they are the party in power, not that they like your positions anymore than the Republican party.
It is not as though Specter was super sympathetic with Republicans on one of the big 3 tent poles in the Republican party--fiscal conservatism, social conservatism, hawkish on foreign affairs/national security. On all of these issues, he is almost a doctrinaire Democrat.
There's room in the Republican party for those who like 2 of 3 or even 1 of 3 (of the 3 tent poles), but if you're moderate-to-liberal on all three of those issues, yeah, the Republican party is not the place for you and good riddance.
UPDATE 5:57pm MST: My friend Dan Kim, for whom I have a lot of respect, addresses Arlen Specter's switch, what it means for the Republican party, & responds a bit to my post.
It's one thing to say that the Republican party holds no appeal to centrists--something which which I heartily disagree--and a whole other thing to say that Specter's defection is emblematic of Republican's lack of appeal to moderates.
Specter moved because Pat Toomey was going to destroy him in the Republican primary. Specter isn't a principled anything--apart from being a principled political survivalist. His quote about not letting the voters of Pennsylvania tarnish his 29 year career by voting him out clearly demonstrates the esteem (or rather, the lack thereof) he holds for the will of the voters. The arrogance manifested by this statement is right in line with what it means to be a career politician.
I didn't like him as a Republican. I like him even less as a Democrat. Arlen Specter is the definition of political opportunism--at least his defection means there is one less opportunist in the Republican party and one more in the Democrat party.
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