Sen. Specter switched from Republican to Democrat because the PA. Republican party was backing people to challenge him and chances are he would have lost the GOP primary. That was a bold move to retain the power that he currently has as a U.S. senator. He probably has a better chance of surviving the Democratic primary than the Republican primary.
While conservatives and non-RINO Republicans seemed to really despise the man, I always wondered how they can criticize him for his voting habits when the people of PA, in state-wide votes, have returned him to office. If anyone is going to be criticized, it should be "the voter" not Sen. Specter.