Pennsylvania Senator Arlen Specter, facing a brutal primary fight for the Republican nomination, announced today that he is switching parties and would run for re-election in 2010 as a Democrat.

"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," Specter said in a statement. "I am ready, willing and anxious to take on all comers and have my candidacy for re-election determined in a general election."

In an afternoon news conference, Specter listed and lamented conservative-led ousters of moderates in Congress and the Senate.

"For the people who are Republicans that just sit by and allow them (conservatives) to continue to dominate the party after they beat (former US Sen. Lincoln) Chafee, cost us the Republican control of the Senate and cost us 34 federal judges," Specter said, "there ought to be a rebellion. There ought to be an uprising."

Specter, 79, was first elected in 1980. The moderate Republican often frustrated more conservative elements of the GOP base, and he was facing possible defeat in the primary because of anger at his vote in favor of President Obama's $787 billion stimulus bill, which conservatives decried.

At 10:25 am today, President Obama was in his daily economic briefing when he was handed a note that said "Spector is announcing he is switching parties," according to a White House official. Obama interrupted his meeting, called Specter at 10:32 am and said "we're thrilled to have you.

"You have my full support," Obama said.

Specter was being challenged by conservative former Lehigh Valley Congressman Pat Toomey, who seized on that vote and others that favored Obama's spending priorities.

Reaction from the Republican party was swift.

"Some in the Republican Party are happy about this. I am not," said Republican National Committee Chairman Michael Steele. "Senator Specter didn't leave the GOP 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."

Toomey came within 17,124 votes -- out of more than 1 million cast -- of defeating Specter in the 2004 primary.

Since then, at least 239,000 Republicans and Independents have switched their party registration to Democrat. There are about 85,000 fewer registered Republicans in the Philadelphia metropolitan region than there were in 2004.

With Specter's party switch, and the pending certification of Al Franken as the new Senator from Minnesota, the Democrats would have a filibuster-proof 60-seat majority in the US Senate.

While Specter noted in his statement that he now finds his political philosophy more in line with Democrats than Republicans, he warned that his vote wasn't automatic.

"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."