Specter's a Democrat. But progressives should take a deep breath and ask if this is, really, such a good thing. In his statement, Specter reiterated his opposition to the Employee Free Choice Act (EFCA), crucial legislation that would help millions of American workers organize unions. If he's reelected as a Democrat, Specter will probably still provide a key vote to maintain the promised Republican filibuster.
It seems to me that the best possible lineup for progressives would be a right-wing Republican taking on a more progressive, pro-union Democrat.
Former congressman Pat Toomey, until recently the President of the Club For Growth, was likely to beat Specter in the Republican primary. Toomey, far too right-wing for Pennsylvania's electorate as a whole, would have likely lost the general election to any number of Democratic candidates. Specter said so much himself.
If Democratic voters decide to elect an actual Democrat in the primary, he will most likely beat Toomey and vote for EFCA. If Specter wins as a Democrat, EFCA faces the same long odds at it would have if he had won as a Republican.