"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. He added, "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. Specter's switch leaves them with 59 Senate seats. Democrat Al Franken is ahead in a marathon recount in Minnesota. If he ultimately 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.
His move comes as Democrats are looking ahead to battles on health care, energy and education.
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 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 Rep. 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 Sen. 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.
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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.
Statement by Senator Arlen Specter
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.
When I supported the stimulus package, I knew that it would not be popular with the Republican Party. But, I saw the stimulus as necessary to lessen the risk of a far more serious recession than we are now experiencing.
Since then, I have traveled the State, talked to Republican leaders and office-holders and my supporters and I have carefully examined public opinion. It has become clear to me that the stimulus vote caused a schism which makes our differences irreconcilable. On this state of the record, I am unwilling to have my twenty-nine year Senate record judged by the Pennsylvania Republican primary electorate. I have not represented the Republican Party. I have represented the people of Pennsylvania.
I have decided to run for re-election in 2010 in the Democratic primary.
I am ready, willing and anxious to take on all comers and have my candidacy for re-election determined in a general election.
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.
I am not making this decision because there are no important and interesting opportunities outside the Senate. I take on this complicated run for re-election because I am deeply concerned about the future of our country and I believe I have a significant contribution to make on many of the key issues of the day, especially medical research. NIH funding has saved or lengthened thousands of lives, including mine, and much more needs to be done. And my seniority is very important to continue to bring important projects vital to Pennsylvania's economy.
I am taking this action now because there are fewer than thirteen months to the 2010 Pennsylvania Primary and there is much to be done in preparation for that election. Upon request, I will return campaign contributions contributed during this cycle.
While each member of the Senate caucuses with his Party, what each of us hopes to accomplish is distinct from his party affiliation. The American people do not care which Party solves the problems confronting our nation. And no Senator, no matter how loyal he is to his Party, should or would put party loyalty above his duty to the state and nation.
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.
Whatever my party affiliation, I will continue to be guided by President Kennedy's statement that sometimes Party asks too much. When it does, I will continue my independent voting and follow my conscience on what I think is best for Pennsylvania and America.
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