The Specter switch

I was going to write a post explaining the logic of Specter's switch to the Dems.  Turns out TPM's Eric Kleefeld has already done the job for me.

So why exactly has Sen. Arlen Specter (R-PA D-PA) switched parties?

It really comes down to electability — specifically electability as a Republican. Specter's own statement acknowledged that his support for the stimulus bill has made his position untenable with the GOP…

Probably the most important point is here is the demographic changes
going on in Specter's home state. Pennsylvania is a closed-primary
state, and the ranks of registered Republicans, the folks eligible to
vote in the GOP primary, shrunk last year.
In 2008, between 150,000 and 200,000 registered GOPers switched to the
Democratic Party in order to vote in the contentious primary between
Hillary Clinton and Barack Obama…

Those people tended to be moderate voters — Specter's people
— and without them he cannot win a primary. But with them staying as
Democrats, he could actually start with a leg-up as a Democrat, just in
case any liberal challenger might try to take him on in the Dem primary.

And the other side of this coin is that the folks who remain as
registered Republicans are now proportionally much more conservative
than the state GOP was before.

Remember that Specter only won his 2004 primary against conservative
challenger Toomey by a 51%-49% margin — and that was with the full
backing of the Bush White House. So if we just made that demographic
adjustment, Pat Toomey would have probably won the 2004 primary with
all other issues being the same. And the stimulus is the final nail.
The stimulus vote, and the lack of a powerful Republican establishment
these days, made a defeat in the primary seemingly inevitable.

As for the Democratic primary, my strong suspicion is that the DSCC and the national Democratic forces will do their very best to clear the field of strong opposition so long as Specter is a reasonably reliable vote for Obama's policies.  That's his only hope of staying in the Senate past 2010, which he clearly wants to do, and was just not going to happen as a Republican.