US Senator Arlen Specter switches parties

Senior Republican Senator Arlen Specter announced that he was becoming a Democrat in a huge boost to President Barack Obama's ability to drive his audacious agenda through the US Congress.

"As the Republican Party has moved farther and farther to the right, I have found myself increasingly at odds with the Republican philosophy and more in line with the philosophy of the Democratic Party," Mr Specter told reporters.

The move gave new life to Mr Specter's embattled 2010 re-election bid and pulled Democrats to 59 seats in the Senate, one shy of the 60 votes needed to override any Republican efforts to kill legislation with parliamentary delay tactics.

Mr Obama's allies will reach that threshold if Democrat Al Franken, who won the vote count in Minnesota's senate contest, prevails over Republican incumbent Norm Coleman, who has appealed the ballot-box results in court.

Coupled with the Democratic majority that can work its will in the House of Representatives, the shift gives Mr Obama immense power to enact the ambitious agenda on which he won the White House in November.

Mr Specter, 79, made clear during a humour-laced press conference at the US Capitol that his decision was shaped by polls giving him "bleak" chances of beating conservative Pat Toomey in Pennsylvania's Republican primary.

"I am unwilling to have my 29-year Senate record judged by the Pennsylvania Republican primary electorate.

I have not represented the Republican Party. I have represented the people of Pennsylvania," he said. Democrats exulted over the defection, which increased their chances of pushing through Mr Obama's ambitious plans for overhauling US health care and tackling climate change with tough, controversial anti-pollution legislation.