McCain, a veteran senator from Arizona, has his party's nomination to run in the November 4 presidential election largely sewn up after top rival Mitt Romney quit on Thursday.

The two remaining Republican candidates, former Arkansas Governor Mike Huckabee and Texas Representative Ron Paul, have not won enough delegates to be considered serious contenders.

McCain, 71, has a solid conservative voting record, but has enraged the key Republican conservative constituency with his campaign finance measures, his stand on immigration reform, and his initial opposition to Bush's huge tax cuts.

A sign of this is unease that Huckabee - who has one-third of the delegates McCain has - was endorsed Friday by James Dobson, founder of the influential Focus on the Family evangelical group.

Dobson cited Huckabee's 'unwavering positions on the social issues, notably the institution of marriage, the importance of faith and the sanctity of human life, resonate deeply with me and with many others'.

Huckabee's name is being mentioned increasingly as possible vice-presidential choice for McCain.

However former Tennessee senator and television star Fred Thompson, who earlier dropped out of the Republican nominating race, formally endorsed McCain on Friday.

To help heal the rift, Republican President George W. Bush appeared at the annual Conservative Political Action Conference in the US capital and tried to whip up support for McCain, his rival in the 2000 Republican presidential primaries.

'Prosperity and peace are in the balance,' Mr Bush told supporters.

'So with confidence in our vision and faith in our values, let us go forward, fight for victory, and keep the White House in 2008,' he said.

Bush has not yet openly backed a candidate, but said he will get behind the Republican party's choice.

McCain, a Vietnam war hero, got a mixed reception when he addressed the CPAC conference on Thursday, with catcalls and boos mixed in with cheers.

Among Democrats, Clinton focused on her universal health care plan as she met voters at a town hall-style rally in Tacoma, Washington.

She also took a swipe at McCain's support for maintaining US troos in Iraq for many years. 'There's no good solution in Iraq, let's be honest here,' she said. 'We are going to make the very best with the bad hand.'

Obama campaigned on Friday in Seattle, while his wife Michelle campaigned for him in Spokane, both in Washington state.

Clinton's campaign said it had refilled its war chest with eight million dollars obtained in online donations, including money from tens of thousands of new donors, since Tuesday's key nominating votes in more than 20 states.

The former first lady was earlier forced to lend five million dollars of her own money to her campaign.

Obama announced on Thursday he had raked in seven million dollars since Tuesday.

The 2008 White House race is already the costliest ever. The Center for Responsive Politics research group estimates the candidates will raise a billion dollars in 2008, a first for a presidential election.

And as Clinton and Obama two chase support from the 2,025 delegates needed to secure the nomination at the party's August convention, every remaining primary now counts.

Washington state and Louisiana hold primary elections Saturday for Republicans and Democrats, while Nebraska holds Democratic primaries, and Kansas holds Republican caucuses.

On Sunday Maine Democrats cast their ballots, and on Tuesday the states of Virginia, Maryland and Washington DC (District of Columbia) hold Republican and Democratic primaries.

A tally by independent pollsters RealClearPolitics on Friday put Clinton only marginally ahead in the delegate count, with 1,077 to Obama's 1,005.

More than 400 delegates are up for grabs in the seven upcoming votes.

Clinton's camp accused Obama of shying away from having debates with her as the stakes rose, after calling for weekly debates up to March 4.

'Senator Obama has refused to debate Senator Clinton on a weekly basis this month, presumably because he is concerned that Senator Clinton is a better debater than he is,' Clinton's communications director Howard Wolfson told reporters.

Obama had said that his main focus was getting out and meeting the voters, and agreed to join televised debates in Texas and Ohio, which have primary nominating votes set for March 4. -- AFP