If you haven't heard by now, Senator Arlen Specter officially left the GOP and has joined the Democratic Party. But why now? Why didn't he do this the first year the Democrats took control like the spineless state legislators in Georgia who switched parties when Republicans took control? It's because he can't really make it out of the Republican primary. In 2004, Specter has the primary well in hand. This time around Toomey already had a slight lead in polling late last year. Once Specter broke rank and voted for the stimulus plan, Toomey's lead skyrocketed. Last I looked, Toomey had a 21% lead over Specter among voters who identified themselves as Republicans. Specter's political future required him to either become an independent or Democrat. Now the Democrats have a theoretical fillibuster-proof 60 votes, but this is the Senate. It's going to be hard to get 60 on your side for anything.
My personal opinion is that Phil Gramm set the proper template on how a politician should switch parties. When he became a Republican, he resigned his seat and reran in the special election. But that shows a sort of cojones that politicians typically just don't have. But such a move hasn't been done before or since as far as I know.