Arlen Specter: 'I do want to serve in a sixth term'
US Republican Senator Arlen Specter is to switch sides and become a Democrat.
The move would give the Democrats 59 votes in the US Senate, just one short of the 60 needed to overturn Republican attempts to block legislation.
The Democrats are expected to pick up their 60th vote when the result of the still-disputed Minnesota Senate race is decided by the courts.
Mr Specter, a moderate, said that since he joined the Republicans in 1980 the party had "moved far to the right".
"I now find my political philosophy more in line with Democrats than Republicans," he said in a statement announcing the switch.
He added: "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."
President Barack Obama, a Democrat, is reported to have called Mr Specter after he was informed of the switch to tell him that he was "thrilled" to have him in his party.
Richard Lister, BBC News, Washington
This is a big prize for President Obama on the eve of his 100th day in office.
There is an element of political survival in Senator Specter's decision. He acknowledged that he could not win re-election in Pennsylvania as a Republican.
Nevertheless, President Obama has given him his full support, and told him in a phone call the Democrats were delighted to have him. The Republican leadership has condemned Senator Specter's decision and vowed to defeat him when he stands for re-election.
Mr Specter, 79, had held secret talks with Mr Obama, along with other senior Democrats including Vice-President Joe Biden, ahead of his decision to leave the Republican Party, according to officials.
The BBC's Richard Lister in Washington says the news has sent shockwaves through Capitol Hill, where the president's power has been blunted by the votes of Senate Republicans.
The move should make it easier for Mr Obama to pursue his ambitious agenda, our correspondent adds.
Mr Specter was facing a tough challenge from conservative Pat Toomey in a primary election to decide who would run as the Republican candidate for Mr Specter's Pennsylvania Senate seat in 2010.
Polls suggested that Mr Specter's decision to vote for the president's economic stimulus package earlier this year had been unpopular with Republican activists in the state.
"I saw the stimulus as necessary to lessen the risk of a far more serious recession than we are now experiencing," Mr Specter said.
Since then, he added, "it has become clear to me that the stimulus vote caused a schism which makes our differences irreconcilable".
If Arlen Specter could have won as a Republican he would have stayed a Republican
Mr Specter may have to fight off a primary challenge from a Democrat if he wants to be the party's Senate candidate in 2010, although he will have the support of senior party officials.
If he does win the Democratic nomination, he is likely to face off against Mr Toomey for the seat.
Including Mr Specter, there will now be 57 Democrats in the Senate. Two independents also vote with the party.
The result of the 2008 Minnesota Senate race is still being fought over in the courts.
State officials and a judge's panel have both awarded victory to Democrat Al Franken, but Republican Norm Coleman has appealed against the decisions to the state's Supreme Court.
A final verdict from the court is due in June.
US MEDIA REACT TO SPECTER'S SWITCH
Specter is one of the better-known senators in America. If you follow politics even casually, you've seen or heard him on the news before. So it's going to register with you that a major Republican senator has decided his party has become too extreme for him. And if you're a Republican, you might wonder if it's become too extreme for you, as well.
Even as we applaud Specter for switching parties, we shouldn't simply concede the primary. Indeed, there needs to be a contested and vigorous primary, especially since Specter's EFCA announcement means he will need pressure on his left, and especially since the primary winner in the increasingly blue state of Pennsylvania has a great shot of defeating someone like Toomey.
We've been systematically making the case since the election that the GOP is now a regional southern party. And what better way to strike home that point than to see a moderate northeastern Republican switch parties, complaining about his party's swing to the far right?
While he'll eat lunch with the Democrats every Tuesday, across the hall from his old party colleagues, he still represents a moderate faction of senators that will form the crux of many legislative negotiations in the upper chamber - signifying a critical vote with enormous power over what language the Senate passes in major policy initiatives.
The real question is - how often will Specter's vote change as a result of this? Specter was already voting with the Democrats on some issues, like the stimulus, and he said in his statement today that he will continue to vote against the Democrats on at least one other high-profile issue, the Employee Free Choice Act. If he goes from voting with the Democrats 40 percent of the time to 60 percent of the time, that is not so terrific for them, particularly if the 60th seat raises expectations and lends credence to Republican claims about the need for divided government.
The most important point here is the demographic changes going on in Specter's home state. Pennsylvania is a closed-primary state, and the ranks of registered Republicans, the folks eligible to vote in the GOP primary, shrunk last year. In 2008, between 150,000 and 200,000 registered GOPers switched to the Democratic Party in order to vote in the contentious primary between Hillary Clinton and Barack Obama... Those people tended to be moderate voters - Specter's people - and without them he cannot win a primary. But with them staying as Democrats, he could actually start with a leg-up as a Democrat, just in case any liberal challenger might try to take him on in the Dem primary. And the other side of this coin is that the folks who remain as registered Republicans are now proportionally much more conservative than the state GOP was before.
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