John McCain today received the backing of California Governor Arnold Schwarzenegger, cementing his position as frontrunner for the Republican nomination before next week's wave of elections in delegate-rich states.

The former action hero film star appeared at a press conference with Mr McCain in Los Angeles and delivered another body blow to the hopes of Mitt Romney, who was reported last night to have balked at spending millions of dollars on TV advertising in the 21 states - including California - which vote on Tuesday.

Mr Schwarzenegger is the second big name prize for Mr McCain in less than 24 hours following his victory in Tuesday's Florida primary. Last night, ahead of a televised debate at the Reagan presidential library in Simi Valley, he was endorsed by Rudy Giuliani.

The former New York mayor pulled out of the race after a disappointing third-placed finish in Florida and described Mr McCain as "the most qualified candidate to be the next commander in chief of the United States - he's an American hero".

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Although popular nationally, both Mr Giuliani and Mr Schwarzenegger are regarded with suspicion by some conservatives. And in the TV debate last night Mr Romney sought to portray his rival as "outside the mainstream of Republican conservative thought", citing Mr McCain's positions on tax, campaign finance reform, illegal immigration and even his recent endorsement by the liberal New York Times newspaper.

Mr McCain, the 71-year-old Arizona Senator, shot back claiming "I'm proud of my conservative record" and noting he had received endorsements from newspapers in Boston, Mr Romney's home town "who know you best". He then added: "I'll guarantee the Arizona Republic will be endorsing me, my friend."

He laid into Mr Romney's record as Massachusetts governor, accusing him of leaving the state with high taxes and debts, as well as imposing a "government mandated" health-care system.

A subsequent spat in a generally gentle debate, where barbs were usually followed by laughter and chuckles, saw Mr Romney accuse Mr McCain of using underhand tactics against him.

The former Massachusetts governor said claims by Mr McCain that he had wanted to pull troops out of Iraq were untrue, before invoking the name of Ronald Reagan - a president whose mantle all the leading Republican candidates have sought to wear.

"I have never, ever supported a specific timetable" for withdrawing troops, said Mr Romney. "It's simply wrong, and the senator knows it. I will not pull our troops out until we have brought success in Iraq." He added that the accusation "falls into the dirty tricks that I think Ronald Reagan would have found reprehensible."

The allegation, which surfaced in Florida, centres around remarks Mr Romney made in April, saying American and Iraqi leaders "have to have a series of timetables and milestones that they speak about".

Mr McCain, who is always at his most comfortable discussing national security issues, refused to back down as he insisted that "of course he said he wanted a timetable" for a withdrawal. But he offered little evidence to sustain his charge and grinned at the sight of Mr Romney - who had attacked him with negative advertising earlier in the campaign - venting anger at him.

Mike Huckabee, the winner in Iowa whose challenge appears to be faltering, was left complaining that "this isn't a two-man race." The Baptist preacher who continues to draw so-called "values voters" away from Mr Romney, added: "If you want to talk conservative credentials, let me get in on that,"

Mr Schwarzenegger, who was in the audience for the debate, listened as Mr McCain praised him for initiatives in which California has set the pace among American states in taking measures to tackle climate change.

Later, Steve Schmidt, a senior adviser to both Mr McCain and Mr Schwarzenegger's campaign manager in his 2006 re-election contest, confirmed the endorsement was on the way. "Governor Schwarzenegger is an exceptional governor and we are honoured that he has decided to endorse Senator McCain," he said.

The two Democratic candidates left in the race, Hillary Clinton and Barack Obama, will go head-to-head for the first time today in a TV debate at the Kodak Theatre in Los Angeles.

Both of them are scrambling for the votes, delegates and endorsement of John Edwards, who yesterday ended his bid for the White House. His backing could yet tilt the balanced of this historically charged - evenly-matched - battle to become either the first woman or the first black man to win a major party nomination.

Yesterday they clashed over a speech Mr Obama made in front of 18,000 people in Denver, Colorado, where he accused Mrs Clinton of being too divisive a figure to win the presidency and repeated his criticisms of her campaign tactics and record.

The Clinton camp said his attacks, which followed the apparent snub he delivered to her during President Bush State of the Union speech, showed he did practice the "new politics" which he preaches.

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