(Evan Vucci ~ Associated Press)
Specter, 79 and seeking a sixth term in 2010, conceded that his chances of winning the Pennsylvania Republican primary next year were bleak in a party grown increasingly conservative. But he cast his decision as one of principle, rather than fueled by political ambition as GOP leaders alleged.
"I have found myself increasingly at odds with the Republican philosophy and more in line with the philosophy of the Democratic Party," he said at a news conference. "I am not prepared to have my 29-year record in the United States Senate decided by the Pennsylvania Republican primary electorate."
Not long after Specter met privately with Republican senators to explain his decision, the party's leader, Sen. Mitch McConnell, said the switch posed a "threat to the country." The issue, he said, "really relates to ... whether or not in the United States of America our people want the majority party to have whatever it wants, without restraint, without a check or balance."
As a result of last fall's elections, Democrats control the White House and have a large majority in the House. Democrat Al Franken is ahead in a recount in Minnesota. If he defeats Republican Norm Coleman, he would become the party's 60th vote -- the number needed to overcome a filibuster that might otherwise block legislation.
Specter, who has a lifelong record of independence, told reporters, "I will not be an automatic 60th vote." As evidence, he pointed out he opposes "card check" legislation to make it easier for workers to form unions, a bill that is organized labor's top priority this year.
Specter was one of only three Republicans in Congress who voted for Obama's economic stimulus bill earlier this year, a measure the senator said was needed to head off the threat of another Great Depression.
Specter called the White House on Tuesday to notify President Obama of his decision to switch. The president called back moments later, according to spokesman Robert Gibbs, to say the Democratic Party was "thrilled to have you."
Several officials, speaking on condition of anonymity, said discussions of a possible switch had reached into the White House in recent days, although Gibbs said he had no details.
Gibbs said Obama would raise money for Specter as well as campaign personally for him if asked.
Specter told reporters at his news conference that Gov. Ed Rendell, a Democrat, had suggested a meeting in Washington for this week at which the party's leadership could formally "endorse my candidacy."
In Pennsylvania, State Rep. Josh Shapiro, a Democrat, swiftly announced he was no longer interested in running for the Senate next year. The only announced Democratic candidate has been Joe Torsella, chairman of the State Board of Education.
Among Republicans, former representative Pat Toomey is expected to run. He had been poised to challenge Specter, who defeated him narrowly in a 2004 primary.
"I welcome Senator Specter and his moderate voice to our diverse caucus," Senate Majority Leader Harry Reid, D-Nev., said in a statement that was a jab at the Republicans.
Other Democrats spread the word on Twitter in a way that reflected surprise. "Specter to switch parties? Wow," said a message sent by Sen. Claire McCaskill of Missouri.
At his news conference, Specter grew animated as he blamed conservatives for helping deliver control of the Senate to Democrats in 2006, making it impossible to confirm numerous judicial appointees of former president George W. Bush.
"They don't make any bones about their willingness to lose the general election if they can purify the party. I don't understand it, but that's what they said," he added.
Ironically, Specter had spoken recently about the importance of a strong Republican presence in the Senate.
"If we lose my seat they have 60 Democrats, they will pass card check, you will have the Obama tax increases, they will carry out his big spending plans. So the 41st Republican, whose name is Arlen Specter, is vital to stopping tax increases, passage of card check and the Obama big spending plans."
Pennsylvania has voted increasingly Democratic in recent elections, and Obama's candidacy in 2008 prompted thousands of voters to switch their registration to his party. Specter said their migration had left the GOP primary electorate "very far to the right."
After nearly six full terms in the Senate, Specter is one of a handful of moderate Republicans left, a politician of remarkable resilience who has maneuvered successfully to protect his seat at home and his seniority rights in Congress.
In line to become chairman of the Senate Judiciary Committee in 2004, he was forced to reassure conservatives he would not attempt to thwart them on Bush's conservative judicial nominees. As a senior lawmaker on the Senate Appropriations Committee, he is responsible for a steady stream of federal projects in his state.
In recent years, he has battled Hodgkin's disease, a cancer of the lymphatic system, but maintains a busy schedule that includes daily games of squash.
Specter was the sixth senator to switch parties in the past 15 years, and the first to leave the Republicans since former senator James Jeffords of Vermont became an independent in 2001. Jeffords' defection gave Democrats control of the Senate. Reid, then the second-ranking Democrat, played a role in that change, as well, offering to give up a committee chairmanship so Jeffords could retain it.
As one of the most senior Republicans in the Senate, Specter held powerful positions on the Judiciary and Appropriations committees. It was not clear how Democrats would calculate his seniority in assigning committee perches.
As recently as late winter, he was asked by a reporter why he had not taken Democrats up on past offers to switch parties.
"Because I am a Republican," he said at the time.
Tuesday's switch was not Specter's first.
He was a Democrat until 1965, when he ran successfully on the Republican ticket for district attorney in Philadelphia.
Associated Press Writers Julie Hirschfeld Davis, Laurie Kellman, Liz Sidoti, Andrew Taylor and Erica Werner contributed to this story from Washington. Peter Jackson contributed from Harrisburg, Pa.