Washington - Senator Arlen Specter, who has served as a Republican in Congress for nearly 30 years, announced Tuesday that he has switched parties, inching the Democrats closer to a super majority in Congress's upper chamber. The Pennsylvania senator's defection brings the Democrats to within one seat of the 60 needed in the 100-seat Senate to overcome Republican attempts to block votes on legislation, and enhances President Barack Obama's ability to advance his ambitious legislative agenda.

Specter, 79, would have faced a strong challenge within the Republican Party for the nomination for the 2010 election after angering conservatives by supporting Obama's 787-billion-dollar economic stimulus package.

"I now find my political philosophy more in line with Democrats than Republicans," Specter said in a statement, pointing out that more then 200,000 Pennsylvania voters switched from Republicans to Democrats last year.

Specter, who has survived two bouts with cancer, announced he now intends to seek the Democratic nomination. "When I supported the stimulus package, I knew that it would not be popular with the Republican Party," he said.

Obama learned of Specter's decision earlier Tuesday morning and telephoned the senator to tell him "he was thrilled to have him as a member of the Democratic Party," White House spokesman Robert Gibbs said.

Whether Democrats can reach the 60-vote threshold will depend on the outcome of a race in Minnesota, which has been deadlocked in a recount battle since the November 4 election. Democrat Al Franken has a razor-thin lead over Republican incumbent Norm Coleman, who has taken the legal manoeuvring to the state's highest court.

Specter had been viewed as a moderate Republican, and he found himself increasingly alienated within its ever growing conservative ranks. During a press conference, however, Specter said he will maintain his independent judgement and cautioned Democrats that his vote will not always be guaranteed.

"I will not be an automatic sixtieth vote," he said.

Specter's decision is yet another major blow to Republicans, who suffered stinging defeats in national elections in 2006 and 2008 under the leadership of former president George W Bush. Republicans were quick to criticize the Specter. Senate Minority Leader Mitch McConnell said the switch harms the balance of power cherished by Americans in government.

"The threat to the country presented by this defection really relates to the issue of whether or not in the United States of America our people want the majority to have whatever it wants without restraint," McConnell said.

Senate Majority Leader Harry Reid welcomed Specter's decision.

"I welcome Senator Specter and his moderate voice to our diverse caucus, and to continuing our open and honest debate about the best way to make life better for the American people," Reid said.

Specter won election to the Senate in 1980 and is now in his fifth term. He had been one of the top ranking Republicans in the Senate until his defection.

But early polls ahead of the 2010 race showed him trailing former congressman Pat Toomey for the party's nomination. A Rassmussen Reports poll published last week showed Toomey with a 21- point lead. Fifty-five per cent of Pennsylvania Republicans viewed Specter unfavourably.

The Democrats enjoy solid control of the House of Representatives, with a 256-178 majority. (dpa)