SPECTER’S STUNNING SWITCH…. Sen. Arlen Specter of Pennsylvania, facing a primary challenge next year he’s almost certain to lose, will switch parties today and become a Democrat. Seriously.

From a press statement issued by the senator’s office about 25 minutes ago:

“I have been a Republican since 1966. I have been working extremely hard for the Party, for its candidates and for the ideals of a Republican Party whose tent is big enough to welcome diverse points of view. While I have been comfortable being a Republican, my Party has not defined who I am. I have taken each issue one at a time and have exercised independent judgment to do what I thought was best for Pennsylvania and the nation.

“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.”

Specter will become the 59th member of the Senate Democratic caucus, which will become 60 once Norm Coleman gives up in Minnesota.

For Specter’s re-election hopes, this makes quite a bit of sense. He was going to lose to Pat Toomey in a GOP primary, and the Republican base in Pennsylvania has grown to actively detest him. Unlike Joe Lieberman’s route in Connecticut, Specter would not have been able to run as an independent after losing the primary. The easiest way for Specter to keep his job is to take the walk across the aisle.

It’s worth noting, though, that Specter has, in recent weeks, tried to move sharply to the right — he even voted for a five-year federal spending freeze earlier this month — obviously to try to curry favor with the party’s base. That had no apparent effect, which no doubt forced Specter to give the Democratic Party another look.

The strategy is not without risks, however. There are already some high-profile, life-long Democrats who’ve been gearing up for next year’s Senate race, and there’s no reason to think they’ll quietly go away simply because of Specter’s switch.

And then, of course, there’s the national scene. Democrats are poised to have a 60-seat majority, though Specter emphasized in his statement, “Unlike Senator Jeffords’ switch which changed party control, I will not be an automatic 60th vote for cloture.”

It’s an important point. In fact, Dems like Ben Nelson aren’t automatic votes for the party, either. It’s tempting to think otherwise, but Specter’s switch will not mean that the majority will be able to necessarily block all efforts at Republican obstructionism.

But Specter’s vote just got considerably more reliable than it was, say, yesterday.

It’s a huge development, and further evidence of a shrinking Republican Party.

Update: Here’s Specter’s full statement, and Chris Cillizza had a good item.

Steve Benen

Follow Steve on Twitter @stevebenen. Steve Benen is a producer at MSNBC's The Rachel Maddow Show. He was the principal contributor to the Washington Monthly's Political Animal blog from August 2008 until January 2012.