US Republican Senator Arlen Specter is to switch sides and become a Democrat¹.

美国共和党参议员Arlen Specter将改投民主党一边。

Mr Specter said the Republicans had "moved to the right"

The move would give the Democrats² 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 decided³ by the courts.

Mr Specter, a moderate（适度的，温和的）, 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.

'Thrilled'

He added: "My change in party affiliation⁴（党籍） 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.

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（变钝） 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（保守的） 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 stimulus⁵ package earlier this year had been unpopular with Republican activists⁶ in the state.

"I saw the stimulus as necessary to lessen⁷ 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 schism⁸（分裂，分立） which makes our differences irreconcilable（不能何解的，不能协调的）".

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 nomination⁹, 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 Supreme¹⁰ Court.

A final verdict（判决） from the court is due in June.