His decision opens the door to the Democratic Party achieving a filibuster-proof majority in the Senate.
Specter, a 79-year-old fifth-term senator, wrote in a statement that he has been a Republican since 1966 and was elected nearly 30 years ago "as part of the Reagan Big Tent." Since then, he said, "the Republican Party has moved far to the right."
At an afternoon news conference, Specter acknowledged that part of the reason he made the decision to switch parties was his grim prospects of surviving Pennsylvania's GOP primary.
"I have traveled the state and surveyed the sentiments of the Republican Party in Pennsylvania and public opinion polls, observed other public opinion polls and have found that the prospects for winning a Republican primary are bleak," he said. "I am not prepared to have my 29-year record in the United States Senate decided by the Pennsylvania Republican primary electorate -- not prepared to have that record decided by that jury, the Pennsylvania Republican primary electorate."
Watch Specter discuss his decision:
Democrats now hold 58 Senate seats, and are likely to have 59 if and when Minnesota Democrat Al Franken is seated, most likely in June. (Former Sen. Norm Coleman is now appealing his apparent loss to Franken to the Minnesota Supreme Court.) Specter's vote would give the party the 60 votes it needs to avoid a Republican filibuster and pass legislation without the support of the minority party.
"What this means, if we are not successful in Minnesota, as you know, is that the Democrats, at least on paper, will have 60 votes," said Senate minority leader Mitch McConnell. "I think the danger of that for the country is that there won't automatically be an ability to restrain the excess that is typically associated with big majorities and single-party rule."
"I will not be changing my own personal independence or my own approach to individual issues," he said. "I will not be an automatic 60th vote." He cited his continuing opposition to the union- and Democrat-backed Employee Free Choice (or Card Check) bill.
The Pennsylvania senator suggested the fallout from his support for the economic stimulus package ultimately led to the decision to switch parties. He was one of just three Republican members of Congress to back the bill.
"It has become clear to me that the stimulus vote caused a schism which makes our differences irreconcilable," he wrote in his statement. Specter said he knew his support for the bill would not be popular in the party but that it was "indispensable to vote aye in order to avoid the possibility of a 1929-type depression."
Specter has not said when he will begin caucusing with the Democrats, though the Associated Press notes that he took a seat on the Democratic side of the dais at a Senate subcommittee hearing today.
Specter had been facing a tough GOP primary fight in Democratic-leaning Pennsylvania. A March poll found that he was trailing Republican Congressman Pat Toomey 41 percent to 27 percent among primary voters in the state. The fact that the state's GOP primary is closed meant that the relatively moderate Specter could not depend on the support of independents or moderates to get him past Toomey.
"Senator Specter's decision today represents the height of political self-preservation," U.S. Senator John Cornyn (R-TX), Chairman of the National Republican Senatorial Committee, said in a statement. "While this presents a short-term disappointment, voters next year will have a clear choice to cast their ballots for a potentially unbridled Democrat super-majority versus the system of checks-and-balances that Americans deserve."
The White House released the above photograph of the call. Specter said at his news conference that Mr. Obama will campaign for him in the Senate primary, and Gibbs said the president is "happy" to raise money for the senator.
"I welcome Senator Specter and his moderate voice to our diverse caucus," Senate Majority Leader Harry Reid said in a statement. Reid will treat Specter as if he were a lifelong Democrat, allowing the senator to keep his seniority in the chamber.
"This is a painful decision," Specter said at his press conference. "I know that I'm disappointing many of my friends and colleagues but, frankly, I have been disappointed by some of the responses. So the disappointment runs in both directions."