Doug Mills/The New York Times Senator Arlen Specter was surrounded by reporters on Tuesday after it was announced that he will switch parties.

Senator Arlen Specter of Pennsylvania said on Tuesday he would switch to the Democratic party, presenting Democrats with a possible 60th vote and the power to break Senate filibusters as they try to advance the Obama administration’s new agenda.

“I’m 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,” Mr. Specter declared in a rather defiant tone at a press conference Tuesday afternoon.

Doug Mills/The New York Times Mr. Specter’s announcement shocked Senate Republicans.

In a statement issued about noon as the Capitol was digesting the stunning turn of events, Mr. Specter said he had concluded that his party had moved too far to the right, a fact demonstrated by the migration of 200,000 Pennsylvania Republicans to the Democratic Party.

“I now find my political philosophy more in line with Democrats than Republicans,” Mr. Specter said in his statement, acknowledging that his decision was certain to disappoint colleagues and supporters.

If Al Franken prevails in his ongoing court case in Minnesota and Mr. Specter begins caucusing with Democrats, Democrats would have 60 votes and the ability to deny Republicans the chance to stall legislation. Mr. Specter was one of only three Republicans to support President Obama’s economic recovery legislation.

The news shocked Senate Republicans, who had been hanging on to their ability to block legislation by a thread. Senator Mitch McConnell of Kentucky, the Republican leader, called an emergency meeting of party leaders who had no forewarning of Mr. Specter’s plans.

On Capitol Hill, Mr. Specter arrived for a vote shortly after noon with his wife, and said he would be lunching in the private Senate dining room rather than joining either of the weekly party policy lunches that were being held.

Democrats were jubilant about the development.

President Obama was handed a note from an aide at 10:25 a.m. on Tuesday during his daily economic briefing. The note, according to a senior administration official, said: “Specter is announcing he is changing parties.”

Seven minutes later, Mr. Obama reached Mr. Specter by telephone. In a brief conversation, the president said: “You have my full support,” according to the official who heard the phone call. The president added that we are “thrilled to have you.”

Mr. Specter spoke with Vice President Joseph R. Biden Jr. slightly before 11 a.m.

According to White House officials, the vice president was at the center of the effort to convince Mr. Specter to change parties. They said that this has been the subject of years of bantering and discussion between the two men – who often sat together while riding the Amtrak train back home – but that the conversation turned earnest after Mr. Biden lobbied Mr. Specter to vote with the White House on the stimulus bill.

One adviser to Mr. Biden said that since that day 10 weeks ago, Mr. Biden and Mr. Specter had spoken 14 times – including six in-person meetings and eight conversations on the telephone. In each case, White House officials said, Mr. Biden argued to Mr. Specter that the Republican Party had increasingly drifted away from Mr. Specter since the election and ideologically, he was closer to the Democratic Party.

They said he did not make mention of Mr. Specter’s political situation in Pennsylvania – where he was facing a primary that could have knocked him off the ballot – because it was not necessary.

White House officials said there was no realistic way to flat out promise Mr. Specter that he would not face a primary in the Democratic Party for the nomination, but noted that there is no Democrat out there in a position to resist the state’s political machine and make a realistic challenge. More than that, White House officials said that they had assured Mr. Specter that he would have the full backing of Mr. Obama should he change parties. They also said that the president would happily campaign for Mr. Specter and raise money for him if that was necessary.

At the White House press briefing on Tuesday afternoon, White House Press Secretary Robert Gibbs confirmed that Mr. Obama pledged his full support to Mr. Specter.

“Full support means full support,” Mr. Gibbs said.

“We will welcome him with open arms,” said Senator Debbie Stabenow, Democrat of Michigan.

Mr. Specter faced a primary challenge from former Republican Congressman Pat Toomey and polls showed him trailing Mr. Toomey. But he had previously resisted overtures to join the Democrats.

“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,” Mr. Specter said in a statement released in the early afternoon. “I now find my political philosophy more in line with Democrats than Republicans.”

He said he has experienced a change of heart since the response to his vote for the stimulus legislation.

“Since then, I have traveled the State, talked to Republican leaders and office-holders and my supporters and I have carefully examined public opinion,” his statement said. “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.”

Mr. Specter, who has a history of finding his own way in the Senate, said he would not be a guaranteed vote for Democratic initiatives and he declared that he would remain opposed to a top labor priority – legislation that would make it easier to unionize American workplaces.

“Whatever my party affiliation, I will continue to be guided by President Kennedy’s statement that sometimes party asks too much,” Mr. Specter said. “When it does, I will continue my independent voting and follow my conscience on what I think is best for Pennsylvania and America.”

Senator John McCain, Republican of Arizona and a man with his own history of breaking with his party, expressed regret and said he had no indication that Mr. Specter would change parties. But Mr. McCain said he understood the reason for Mr. Specter’s shift: “It’s pretty obvious the polls show him well behind his primary opponent.”

Michael Steele, the chairman of the Republican National Committee, did not mince words about the senator, saying Mr. Specter “didn’t leave the G.O.P. 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 Senator Specter in 2010, assuming the Democrats don’t do it first.”

But Senator Olympia Snowe of Maine, who also supported the Obama administration’s economic stimulus legislation, said Mr. Specter’s decision reflected the increasingly inhospitable climate in the Republican party for moderates.

“On the national level of the Republican Party, we haven’t certainly heard warm, encouraging words about how they view moderates, either you are with us or against us,” Ms. Snowe said. She said national Republican leaders were not grasping that “political diversity makes a party stronger and ultimately we are heading to having the smallest political tent in history for any political party the way things are unfolding.”

Senator Jon Kyl of Arizona, the No. 2 Republican in the Senate, called the decision “a real problem.”

Mr. Specter, who has had serious health problems in recent years, remains active on a variety of major issues and has been a leading advocate for increased funding for health care research.

Democrats called the decision a game-changer. “It helps on everything,” said Senator Dianne Feinstein, Democrat of California. “This is a substantial change.”

Democrats said they made no promises to Mr. Specter about committee positions or other incentives to switch, but the party can provide significant campaign support and deter other Democratic candidates from running against him in the primary next year.

The turnabout was reminiscent of the decision in 2001 by Senator Jim Jeffords of Vermont to leave the Republican party and become an independent, handing control of the Senate back to Democrats just as President George W. Bush’s first term was beginning. The Jeffords switch flipped party control but some Democrats said Mr. Specter’s move could be just as consequential given the Senate’s recent struggles with filibusters.

“Specter’s decision could be more consequential because it came just as the Senate was beginning work on health care reform,” said Senator Ron Wyden, Democrat of Oregon. “Specter’s decision is a big impact decision.”

Mr. Specter’s move to the Democratic column is likely to have a chilling effect on other potential Democratic candidates for the Senate. So far, Joseph Torsella, former head of the National Constitution Center and a former deputy mayor of Philadelphia, is the only Democrat to have declared his candidacy.

Others with higher name recognition seem to have been holding back to see how the field would shape up. Even before Mr. Specter announced his switch today, Representative Allyson Schwartz, a Democrat representing parts of Philadelphia and the nearby suburbs, had told The New York Times she was unlikely to make the run. Other possibilities, including Representatives Patrick Murphy and Joe Sestak, had also stayed mum.

Gov. Edward G. Rendell, a Democrat, and Mr. Biden, both of them long-time friends of Mr. Specter, had urged him to switch parties several weeks ago but Mr. Specter declined. Mr. Rendell said in a recent interview that he had promised Mr. Specter that if he became a Democrat, he would help him raise money; Mr. Specter joked that if he became a Democrat, he wouldn’t need Mr. Rendell’s help on that front.

There had been speculation in Pennsylvania political circles that something was afoot because Mr. Torsella, a close colleague of Mr. Rendell, said little about Mr. Specter when he announced his candidacy.

But Mr. Specter put the kibosh on talk that he might leave the Republican Party and become either a Democrat or an independent, insisting, though without much evidence, that there was room in the Republican Party for moderates.

The move brings Mr. Specter full circle with his earlier political leanings. He was a registered Democrat when he first ran for district attorney of Philadelphia in the mid-1960s, though he ran on the Republican line.

Read Mr. Specter’s full statement.

Room for Debate: What Kind of Democrat Will Specter Be?

Read The Lede Blog: Specter’s Plan to Rein In the Presidency.

View an interactive timeline of Mr. Specter’s political career.

Jeff Zeleny, David M. Herzenhorn, Robert Pear and Adam Nagourney contributed reporting from Washington, and Katharine Q. Seelye from New York.