According to Chris Cillizza, Specter has issued a statement. “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.”

Specter 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.”

But, of course, attributing to Arlen Specter a coherent or consistent “political philosophy” is giving him more credit than he has ever deserved. Specter, who originally switched in from being a lifelong Democrat to a Republican in a 1965 move to be elected Philadelphia’s district attorney, is the quintessential opportunist. Though he was elected on Ronald Reagan’s coat tails in 1980, he has always been in business for himself. As former Congressman Joe Hoeffel, his Democratic opponent for the Senate in 2004, put it to me in an interview, “It’s very hard to run against Arlen on the issues because he is on both sides of every question.”

As in 1965, this is a move dictated solely by political survival not principle. In 2004, he survived a stiff primary challenge from conservative Pat Toomey and won only by virtue of the strong support given him by George W. Bush and former Senate colleague Rick Santorum. But only hours after that narrow primary win, Specter began distancing himself from both of them. Given the fact that there would be no such help forthcoming in 2010 against Toomey, who jumped into the race after Specter voted for President Obama’s stimulus boondoggle, it was a given that Specter would lose the Republican nomination and his Senate career would come to an end. So rather than go down as a Republican, Specter will jump ship.

This is bad news for a number of Pennsylvania Democrats who were hoping to be able to run against Toomey in November 2010 and figured that the weakened state of the Republicans would ensure a victory. We can assume this means that almost all of the Democrats who were exploring a Senate race will back out, not wishing to anger a party establishment that is happy to have a champion fundraiser like Specter on their side for once. But my bet is that Specter will not go unopposed in a Democratic primary next year. There’s bound to be at least one left-winger in the state that will count on the moveon.org crowd to get behind a challenge to a man whom many leftists still hate for his questioning of Anita Hill.

Still, you have to assume that Specter will be a huge favorite not only to win the Democratic nomination, but also to beat Toomey in November. Pennsylvania has been trending more and more to the left in recent elections and Specter knows this.

This is very good news for President Obama who will presumably now have his 60th vote in the Senate when and if Al Franken is seated for Minnesota. But Obama and other Democrats should be prepared for a difficult marriage with Specter. He is bound to be as feckless and faithless a Democrat as he was a Republican.