Democratic front-runner Barack Obama's White House bid received a fresh momentum on Friday as he secured the support of three more superdelegates, including a former backer of his rival Hillary Clinton.

New Jersey representative Donald Payne, who had earlier backed Clinton for President, defected to the African-American Senator, while previously uncommitted lawmaker Peter DeFazio of Oregon also pledged his backing.

The American Federation of Government Employees threw its weight behind Obama and its President John Gage, an uncommitted superdelegate to the Democrat National Convention, said he is also personally endorsing the Illinois lawmaker.

"Our people, I think, recognise the enthusiasm and vitality behind Senator Obama's campaign," Gage said.

According to an AP tally, the superdelegate count is now Clinton, 271.5, and Obama, 266, a huge shift since the days when the former first lady boasted about a 60-plus lead in the crucial voting bloc.

Obama's race for Democrat presidential nomination had received a boost on Tuesday, when he clinched a convincing victory in the North Carolina primary and held Clinton to a narrow win in Indiana.

While Clinton is refusing to concede defeat and is hoping that a victory in Tuesday's West Virginia primary will keep her stuttering campaign alive, Obama is starting to focus instead on his Republican opponent John McCain, ABC News reported.

Obama's team is considering using some of his campaign cash to fund ads against McCain, it said.

Clinton, campaigning on friendly turf in West Virginia, said she had faced similar pressure to withdraw before she went on to win New Hampshire, Ohio, Texas and Pennsylvania's primaries.

"I'm running to be president of all 50 states," she said. "I think we ought to keep this going so the people of West Virginia's voices are heard."

Obama, who holds a commanding lead in the number of pledged delegates awarded from primaries and caucuses, today also had a virtual endorsement from former Democratic White House hopeful Senator John Edwards.

"I think (Hillary Clinton) has made a very strong case for her candidacy, but the problem is the numbers," Edwards told NBC television. "Let's just assume that Barack is the nominee because it's headed in that direction."

Edwards said he thought Obama has a better chance to beat McCain but stopped short of issuing an endorsement for him.

"I think Americans are looking for someone who will fight for them every day ... and I think that Obama will do that," he said.