While he was able to shrug off the taunts of "Benedict Arlen," and "Judas," Sen. Arlen Specter found that "changing parties involved a high level of trauma. It wasn't like changing religions, but there were elements of arguable disloyalty, and opportunism that rubbed me the wrong way."

That was nearly a half-century ago, when, as he recounts in his autobiography, "Passion for Truth," he bucked the entrenched machine of his longtime party, a party that had spurned him, to become Philadelphia's district attorney as the nominee of the Republicans. Yesterday, Mr. Specter's political path came full circle as the 79-year-old stunned his Senate colleagues with the announcement that he would seek a sixth Senate term as a Democrat.

"I am not prepared to have my 29-year record in the United States Senate decided by the Republican primary electorate, not prepared to have that record decided by that jury," Mr. Specter said at a Capitol Hill news conference.

After months of insisting that he would run again as a Republican in the face of a renewed challenge from the conservative wing of his party, Mr. Specter said that daunting poll results had led him to decide over the weekend to accept the longstanding courtship of senior Democrats to return to their fold.

The move reflected and reinforced a Democratic tide in Pennsylvania politics exemplified by President Barack Obama's big win in 2008 and the gains in congressional races the party has scored over the last two election cycles.

But more immediately, it created the potential for a filibuster-proof Senate majority that could give the Democrats unfettered control of the federal government as Congress deals with the Obama administration's ambitious agenda. With his switch and the anticipated victory of Al Franken in the protracted Minnesota Senate race, the Democrats would have the 60 votes needed to turn aside GOP opposition to legislation or Obama nominees to the judiciary or any other posts requiring Senate confirmation.

Mr. Specter insisted, however, that the political reality might be more complicated than the legislative arithmetic might suggest. He said, for instance, that he would continue to oppose legislation designed to make it easier for unions to organize new work places.

"I will not be an automatic 60th vote," he said.

Mr. Specter faced an uphill battle to retain the GOP nomination next year in the face of challenges from Pat Toomey, the former congressman who just missed unseating him in 2004, and from another conservative, Peg Luksik. The long-simmering conservative opposition he had managed to survive for most of his career came to a boil earlier this year with the vote that made him one of only three Republicans to support the stimulus package sought by the Obama administration.

The president was informed of Mr. Specter's decision in a note passed to him during his morning economic briefing. According to a White House official, the two talked by phone at 10:32 a.m. and the president told Mr. Specter that he was "thrilled" to welcome him to the Democratic Party.

In his daily briefing shortly thereafter, Obama press secretary Robert Gibbs said the new Democrat had the president's "full support."

Asked about the possibility of a Democratic primary, he reiterated, "Full support means full support."

In a subsequent news conference, Mr. Specter said that the president told him that he was prepared to campaign for him next year. Gov. Ed Rendell had no immediate comment on the switch, but Mr. Specter, who informed the governor of his decision yesterday morning, said that Mr. Rendell would travel to Washington today to publicly demonstrate his backing.

The backing of the president and the governor should go a long way toward clearing the Democratic field for the party's new convert, although some Democrats insisted that that was not a certainty.

One rumored candidate, state Rep. Josh Shapiro of Montgomery County, immediately dropped talk of running. One of only two declared official Democratic candidates, Philadelphia's Joe Torsella, said he is remaining in the race. But Mr. Torsella is a close political ally of the Democratic governor.

State Rep. Bill Kortz, D-Dravosburg, who only got into the Democratic U.S. Senate race last week, said he is "evaluating what to do, but I am leaning toward staying in the race."

Another possible Democratic contender, U.S. Rep. Joe Sestak of Delaware County, told MSNBC Mr. Specter is "shifting parties because he found it hard to run against someone. So I'm interested to see what he's running for."

Bill George, the president of the state AFL-CIO, issued a statement welcoming Mr. Specter's decision, but said that the labor federation, which has endorsed him in the past, would withhold a final 2010 judgment while continuing to try to change his mind on the Employee Free Choice Act.

Republican reaction ranged from regret to outrage.

Noting Mr. Specter's previous vows to remain in the GOP, the Toomey campaign said in a statement that, "What Pennsylvanians must now ask themselves is whether Sen. Specter is in fact devoted to any principle other than his own re-election."

Ms. Luksik, an anti-abortion activist from Johnstown, said, "He's much more comfortable on the Democratic side than he ever was on the Republican side."

Former Sen. Rick Santorum, who stirred Republican criticism five years ago by supporting Mr. Specter over Mr. Toomey in their hard-fought primary, told Politico.com that Mr. Specter "explained his reasoning to me. I told him I was deeply disappointed that he felt he had to do it."

For many Republicans -- such as GOP political consultant John Brabender -- Mr. Specter's motives seemed clear. "This is not a profile in courage, this is a profile in saving my political career," he said.

But former GOP national committeewoman Elsie Hillman -- who hosted a recent fundraiser for the senator in her Squirrel Hill home, stood by her fellow moderate.

"I believe that he is the brightest member of the United States Senate and I will continue to support him and will vote for him in next year's general election," she said.

Recent public polling suggests that Mr. Toomey is the stronger of the remaining GOP candidates, but Mr. Specter's decision creates the possibility of a new candidacy from the more moderate wing of the GOP.

Terry Madonna, who directs the Franklin & Marshall College poll, suggested that the race might now be attractive to figures such as Rep. Jim Gerlach, a Chester County Republican who has been considering a bid for governor.

Mr. Specter said that he made his decision over the weekend after his own pollster confirmed a pattern of public surveys suggesting that he could not win the nomination of an increasingly conservative Pennsylvania Republican Party. He said he consulted his wife and his son, Shanin, his close political confidante, over the weekend before making what he called a painful decision.

He met with Senate Democratic leader Harry Reid on Monday at about 6 p.m. to tell him that his caucus was about to get a new member. Mr. Specter said he would change his party registration when the voter rolls are reopened after this year's primary.

Mr. Specter said he and Mr. Reid had agreed on "the fair approach," whereby his seniority among his new colleagues would include all of his years of Senate service. That means that he will not chair a major committee in this session of Congress but that he will be near the top of the seniority list on crucial panels including Judiciary and Appropriations.

His next stop was the Senate office of Sen. Mitch McConnell, the Republican leader whose caucus was further weakened by the loss of the crucial vote.

"The threat to the country presented by this defection really relates to the issue of whether or not ... our people want the majority to have whatever it wants without restraint or without a check and a balance," the GOP leader said.

The round of calls continued yesterday morning as Mr. Specter spread the closely held news to Mr. Rendell, Vice President Joe Biden and Sen. Bob Casey, all of whom had publicly urged him to cross the party aisle.

At his news conference yesterday, Mr. Specter recalled that in urging him to switch parties, the governor, the most prolific fundraiser in the state's political history, had promised to raise money for him.

"I responded that if I became a Democrat, I wouldn't need him to help me raise money," Mr. Specter said.

"I've changed my mind on that," he added.