Straight from the horse's mouth on Specter2010.com:
Since my election in 1980, as part of the Reagan Big Tent, the Republican Party has moved far to the right. Last year, more than 200,000 Republicans in Pennsylvania changed their registration to become Democrats. I now find my political philosophy more in line with Democrats than Republicans…I have decided to run for re-election in 2010 in the Democratic primary.
Self-preservation at its finest. My knee-jerk reaction is that he will cakewalk through 2010 as a Democrat. More on this soon.