Or rather, he’s finally making it official. Human Events broke the news, now WaPo confirms:

“I have decided to run for re-election in 2010 in the Democratic primary,” said Specter in a statement. “I am ready, willing and anxious to take on all comers and have my candidacy for re-election determined in a general election.”

He added: “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.”

That’s a weaselly lie, and therefore a perfect note for him to depart on. He’d have happily run for re-election as a Republican if not for Toomey getting into the race and quickly jumping out to a 21-point lead. Specter tried to make it an open primary so that the left might rescue him but couldn’t, and Pennsylvania’s election laws prevented him from doing what Lieberman did to Lamont three years ago: In PA, if you compete in a primary and lose, you’re done. No independent candidacy. So his choice, essentially, was either to switch to an independent now and skip the primary or go the whole nine yards by becoming a Democrat, giving the left a presumptive filibuster-proof majority (once Franken is seated), and extracting whatever concessions he could from them in return, e.g. committee chairmanships, DNC fundraising, etc. The Hill actually kinda sorta predicted this last month. It’s pure self-preservation on Specter’s part, expecting that he’ll be able to handle Toomey easily in the general when Democrats and indies can push him through.

Three quick thoughts. One: Does this mean he’s going to reverse himself on Card Check? I’m guessing yes. Two: Does this mean the Democrats will drop their threat to nuke the filibuster on health care? Hard to say since Blue Dogs like Ben Nelson could defect and deprive them of the 60th vote. Third: Will a lefty challenger jump into the Democratic primary now and challenge Specter as, irony of ironies, a DINO?

You know who I bet feels pretty stupid right now? John Cornyn.

Update: Ben Smith says Biden was “deeply involved” in the switch. Here’s Specter’s full statement. Interesting:

I deeply regret that I will be disappointing many friends and supporters. I can understand their disappointment. I am also disappointed that so many in the Party I have worked for for more than four decades do not want me to be their candidate. It is very painful on both sides. I thank specially Senators McConnell and Cornyn for their forbearance…

My change in party affiliation does not mean that I will be a party-line voter any more for the Democrats that I have been for the Republicans. Unlike Senator Jeffords’ switch which changed party control, I will not be an automatic 60th vote for cloture. For example, my position on Employees Free Choice (Card Check) will not change.

Update (Ed): I’m in the good-riddance category here.  Normally I argue for a big tent and the need to woo moderates by focusing on core values.  Specter betrayed those values in his Porkulus vote and cloture cave.  He could have forced Obama, Pelosi, and Reid to start negotiating in good faith with his Republican colleagues, but instead allowed them to shove a bad bill down their throats.

Update: You’ll be pleased to know that Snarlin’ Arlen has The One’s “full support.” Meanwhile, here’s just how much of a weaselly liar he is. From a March 17 interview with The Hill:

I am staying a Republican because I think I have an important role, a more important role, to play there. The United States very desperately needs a two-party system. That’s the basis of politics in America. I’m afraid we are becoming a one-party system, with Republicans becoming just a regional party with so little representation of the northeast or in the middle atlantic. I think as a governmental matter, it is very important to have a check and balance. That’s a very important principle in the operation of our government. In the constitution on Separation of powers.

Update: Michael Steele lays it on the line:

Some in the Republican Party are happy about this. I am not. Let’s be honest-Senator Specter didn’t leave the GOP based on principles of any kind. He left to further his personal political interests because he knew that he was going to lose a Republican primary due to his left-wing voting record. Republicans look forward to beating Sen. Specter in 2010, assuming the Democrats don’t do it first.

Update: Politico says Specter was talking to the Dems about this for months and that the final straw was indeed the polls showing him getting crushed by Toomey. Question: Are any other Republicans thinking of switching? Hmmmm:

Sen. Olympia Snowe (R-Maine) a fellow moderate, didn’t seem suprised. On the national level, she says, “you haven’t certainly heard warm encouraging words of how they [Republicans] view moderates. Either you are with us or against us.”

“Ultimately we’re heading to having the smallest political tent in history they way things are unfolding,” Snowe said. “We should have learned from the 2006 election, which I was a party of. I happened to win with 74% of the vote in a blue collar state but no one asked me how did you do it. Seems to me that would have been the first question that would have come from the Republican party to find out so we could avoid further losses.”

Update: The most discouraging thing about all this may be that the GOP leadership appears to have been caught totally by surprise, even though (a) per Politico, Specter’s been talking to the Dems for months, and (b) per Toomey’s polling, we’ve been speculating about a Specter switch for weeks now. How could they have been caught flat-footed on this?

Update: Grahamnesty wants in on some of Snowe’s “viva moderates!” action.

“I don’t want to be a member of the Club for Growth,” said Graham. “I want to be a member of a vibrant national Republican party that can attract people from all corners of the country — and we can govern the country from a center-right perspective.”

“As Republicans, we got a problem,” he said.

Counters Jim DeMint, whose impending endorsement of Toomey might have pushed Specter over the edge, “I would rather have 30 Republicans in the Senate who really believe in principles of limited government, free markets, free people, than to have 60 that don’t have a set of beliefs.”