Arlen Specter (D-PA):

When I supported the stimulus package, I knew that it would not be popular with the Republican Party. But, I saw the stimulus as necessary to lessen the risk of a far more serious recession than we are now experiencing.

Since then, I have traveled the State, talked to Republican leaders and office-holders and my supporters and I have carefully examined public opinion. It has become clear to me that the stimulus vote caused a schism which makes our differences irreconcilable. On this state of the record, I am unwilling to have my twenty-nine year Senate record judged by the Pennsylvania Republican primary electorate. I have not represented the Republican Party. I have represented the people of Pennsylvania.

I have decided to run for re-election in 2010 in the Democratic primary.

This is really good move for just about everybody.  The Democrats are happy to see their ranks bolstered to 59 (and soon to be 60) in the Senate, Arlen Specter no longer has to worry about losing his primary bid, and the people of Pennsylvania will most likely get to keep their Senator.

I said almost everybody because this is definitely not good for the Republican Party.  They can whine and pretend that they don’t care, acting like they’re glad to see him go (see Malkin), but ultimately what this blares out in the grandest fashion is: There’s no room in the Republican Party for moderates.

Nate Silver is calling this the Republican Death Spiral:

Thus the Republicans [...] are in something of a death spiral. The more conservative [...] their message becomes, the more they alienate non-base Republicans. But the more they alienate non-base Republicans, the fewer of them are left to worry about appeasing. Thus, their message becomes continually more appealing to the base — but more conservative, partisan, and strident to the rest of us. And the process loops back upon itself.

But what does this actually mean for the sausage in Congress?  Probably not much.  Specter will eventually slide a little to the left because he no longer has to pretend he’s more conservative than he is, but he’s no Bernie Sanders (or Barack Obama, for that matter).

I’m thinking he’ll end up somewhere between a Ben Nelson and an Evan Bayh, which, on the whole, is a net gain.