While it may seem a bit surprising at first, it is not out of character for him. He has never been part of the Republican base--his voice has long been one of the voices of moderation in the party. Also, it appears that the polling for the upcomming primary was unfavorable to him. Five years ago he had a very tough fight against a an extreme conservative and a close general election too. Probably the only reason he was able to prevail in the general election was because he is so popular with many Democrats and almost al Independents here in PA. It was certainly a pragmatic move, and Specter is, above all else, a pragmatic politician.
But it also makes sense philosophically.
“I now find my political philosophy more in line with Democrats than Republicans,” Mr. Specter said, acknowledging that his decision was certain to disappoint colleagues and supporters.
Welcome to the ship that isn't sinking, Arlen.