WASHINGTON -Pennsylvania Sen. Arlen Specter, one of the few moderate Republicans left in Congress, announced Tuesday that he was switching parties, a major gain for Democrats in their quest for a filibuster-proof majority in the Senate to propel President Barack Obama's ambitious agenda.
Specter's decision was also another log on the bonfire that is eating away at the GOP as a national political force. He has been one of only a handful of Republicans able to win elections while rejecting the strict anti-abortion, anti-spending, pro-gun-rights conservatism that now dominates the party.
Facing a stiff primary challenge from a conservative former House member, Pat Toomey, Specter on Tuesday said bluntly that he concluded he could win re-election only as a Democrat.
The announcement caught most of official Washington by surprise, stirring jubilation among Democrats and sending Republicans scurrying to insist it was a matter of local Pennsylvania politics, not a sign of a change in national politics.
Nonetheless, Specter's change of party affiliation reflected both his calculation of the present situation in his home state and what Senate Majority Leader Harry Reid of Nevada and Senate Majority Whip Dick Durbin of Illinois described as a five-year effort by Democratic leaders to win him over.
Before going public with his decision, Specter spoke to and won commitments of support from Obama and Reid. Obama said that, if asked, he would campaign for Specter in Pennsylvania's Democratic primary - a venture into local politics that presidents usually avoid because it can lead to bruised feelings and divisions within the party.
The White House commitment, along with Reid's agreement not to penalize Specter in committee ranking, pointed to Specter's potential value to the Democrats.
If, as expected, a contested Minnesota Senate election is decided in favor of Democrat Al Franken over former Republican Sen. Norm Coleman, Specter would give the Democrats 58 members in the Senate. Adding two independents who usually align with them would create the 60-vote margin required to block a filibuster - the minority party's most powerful tool for stalling legislation.
That does not guarantee a sweeping change in the balance of power in the Senate.
"I will not be an automatic 60th vote," Specter declared.
Still, he has been a reliable ally for Democrats on such matters as health research funding and abortion rights. According to an analysis by Congressional Quarterly, Specter voted with the majority of the GOP in only 62 percent of party-line votes.
He demonstrated the power of his vote early this year when he provided one of only three Republican votes for Obama's economic stimulus bill.
At the White House, Press Secretary Robert Gibbs said there had been no deal over the switch, but he added: "If the president is asked to raise money for Senator Specter, we're happy to do it. If the president is asked to campaign for Senator Specter, we'll be happy to do it. As the president told Senator Specter on the phone, he has our full support, and we're thrilled to have him."
As Specter pondered the decision to change parties, the political bind he found himself in was a measure of how much American politics has become polarized by region. The GOP once had a robust wing of moderates - Rockefeller Republicans or "Gypsy Moths" who hailed mostly from the Northeast. Now, the party's regional base is largely concentrated in the South and dominated by more rigidly ideological conservatives.
That shift is part of what Specter said had driven him from the party. "As the Republican party has moved further and further to the right, I have found myself increasingly at odds with the Republican philosophy," he said.
But Specter was primed for a switch six years ago when he faced a brutal primary challenge from Toomey, and eked out a victory - thanks in part to a big, 11th-hour push from President George W. Bush. And it isn't the first time he has switched party affiliation. He had been a Democrat until he changed his registration to Republican in 1965 to run for Philadelphia district attorney. He was first elected to the Senate in 1980.
Even if he gained the Republican nomination in 2010 - and a recent poll of Republicans showed he was trailing Toomey by some 21 percentage points - his prospects for winning the general election are grim because there's been a surge in Democratic voter registration since he last ran: Some 200,000 Republicans have switched registration.
Specter, who is 79 years old, was unabashed in acknowledging that his decision was impelled by poll results late last week.
"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," he said.
That led Senate GOP leader Mitch McConnell of Kentucky to insist that the decision was a reflection of Pennsylvania politics, not a statement on the national state of the party.
Republican National Committee Chairman and former Maryland Lt. Gov. Michael Steele was defiant.
"Republicans look forward to beating Senator Specter in 2010, assuming the Democrats don't do it first."
The Associated Press contributed to this article.