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John McCain was last night on course to become the Republican’s presidential nominee after a decisive victory in the key state of California crowned a night of cross-country victories.

His chief rival, Mitt Romney, vowed to fight on despite a disappointing night that saw the social conservative vote split between him and Mike Huckabee, who won five states in the South.

"Although I've never minded the role of the underdog, we must get used to the idea that we are the Republican Party frontrunner for the nomination of president of the United States," a relaxed and smiling Mr McCain told cheering supporters at his headquarters in Phoenix. "And I don't really mind it one bit."

Mr Romney picked up seven states but lost out on California, where he needed a win to sustain momentum in his challenge for the nomination.

In a upbeat speech in Boston, Mr Romney told supporters: "One thing that is clear is that this campaign is going on." He is still attracting support from conservative voters hostile to Mr McCain's maverick views on immigration and climate change.

Mr Huckabee, who had earlier appeared to team up with Mr McCain to deny Mr Romney victory in west Virginia, said: "I've got to say that Mitt Romney was right about one thing — this is a two-man race. He was just wrong about who the other man in the race was. It's me, not him."

Mr Romney’s campaign accused Mr McCain of "cutting a backroom deal" after the Arizona senator's team allegedly told supporters to vote in favour of Mr Huckabee in West Virginia.

Even before California was called for Mr McCain, the 71-year-old triumphantly claimed his mantle as the Republican frontrunner as he counted up a huge haul of delegates bulked up by victories in a string of winner-takes-all states including New York, New Jersey, Connecticut and Delaware.

Mr McCain ended the night with a massive haul of 482 delegates, against 168 for Mr Romney and 117 for Mr Huckabee.

His speech ended with a call for unity for a party which had been "my political home for half a century", promising to ensure that the "conservative philosophy and principles" defeats the Democrats.

Although many on the right now appear ready to accept Mr McCain as their best bet to beat the Democrats, he has faced bitter opposition from conservative detractors led by attack-dog radio show host, Rush Limbaugh, seeking to derail his White House bid.

Mr McCain had warm words for the Baptist preacher, Mr Huckabee, who is sometimes mentioned as a possible vice-presidential candidate.

"Not for the first time, he surprised the rest of us and proved again his exceptional skills as a campaigner and the extraordinary commitment and determination of the people who believe so passionately in him," said Mr McCain. "I salute you. I salute Governor Huckabee."

He also managed to compliment the competitive spirit of Mr Romney - a man for whom he is believed to harbour deep enmity - saying: "He and I having been going pretty hard for the last few weeks. He is a pretty tough competitor."

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