WASHINGTON – 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, 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 only win re-election 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 up the senator’s potential value to the Democrats.

If, as expected, a contested U.S. Senate election in Minnesota is decided in favor of Democrat Al Franken over 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. For example, Specter on Tuesday restated his opposition to organized labor’s top priority, a bill to make it easier to unionize workplaces. And under Republican presidents, he has supported conservative judicial nominees.

“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, eleventh-hour push from President George W. Bush.

His prospects for re-election in 2010 were even grimmer: A recent poll of Republicans found he was trailing Toomey by some 21 percentage points. What is more, his general election prospects have been clouded by a surge in Democratic voter registration since he last ran: Some 200,000 Republicans have switched registration.

“It’s a bluer state than it was,” said Sen. Bob Casey, D-Pa. “It’s the nature of these changing times we live in.”

Specter 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 lead Senate GOP leader Mitch McConnell, R-Ky., to insist that the decision was a reflection of Pennsylvania politics, not a statement on the national state of the party.

Specter’s choice was an abrupt switch from just a few weeks ago, when he insisted in an interview that he was running as a Republican to keep Democratic power in check.

“The one thing standing between a Democratic steamroller and the American people are the 41 votes in the Senate,” he said. “If I’m not in the Senate, they’ll be 40 and the other side will have 60; there will be no checks and balance.”

Specter was first elected to the Senate in 1980, part of the GOP landslide that put Ronald Reagan in the White House and handed the party control of the Senate. Now 79 years old, he has battled several rounds of cancer.

Obama learned of Specter’s decision at 10:25 a.m. EDT Tuesday, when he was handed a note during a meeting on economic policy. He reached Specter by phone shortly thereafter and warmly welcomed him to the party.

“The president is quite pleased,” Gibbs said. “And that is the understatement of the day.”

Earlier, Specter called Vice President Joe Biden, a close friend from their years of service together in the Senate. They often traveled on the same Amtrak train from Washington, with Biden stopping in Wilmington, Del., and Specter going on to Philadelphia. Biden never made a secret of the fact that he’s wanted Specter to cross the political aisle and has been in frequent contact with him since he became vice president

It was not as dramatic a party switch as in 2001, when GOP Sen. Jim Jeffords of Vermont quit the party to become an independent, throwing his weight on the side of the Democrats. That tilted the party balance in the Senate to a 51-49 Democratic advantage, giving them new leverage to battle the Bush administration.

Reid went out of his way to lowball expectations of what a difference Specter’s vote will make. “We’ve not always agreed on every issue in the past, or will we in the future,” he said.

On Obama’s signature health care initiative, Democratic leaders had already decided to use a procedural maneuver that would avoid a GOP filibuster even without Specter.

The biggest impact may be on Republicans’ morale. The party has struggled to recover from its drubbing in the 2008 elections, and it just lost a special election in a Republican-leaning House district in upstate New York. The party is also expected to lose the Minnesota Senate race in a matter of weeks.

And many lawmakers doubt their ability to hold onto the Pennsylvania seat unless the GOP fields a candidate with broader appeal than Toomey.

“We have to broaden this party,” said Sen. Lindsey Graham, R-S.C. “Pat Toomey is a fine fellow but he can’t win.”

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