Smash hit

Mr McCain, who carries wounds inflicted on him as a prisoner in the Vietnam war, said: "If we're going to get the high ground in this world and we're going to be the America that we have cherished and loved for more than 200 years, we're not going to torture people."

He has tenaciously fought to reinvigorate his campaign – badly damaged by supporting legal rights for undocumented workers – and last night he won applause with a principled defence of the military surge in Iraq, winning exchanges with libertarian wild card Ron Paul, who wants to withdraw all US forces from overseas.

Mr Giuliani more or less survived tricky questions posed by YouTube videos about his stance on gun ownership and abortion, but then had to deny allegations that he had obscured thousands of dollars in taxpayer costs for his security detail as New York mayor at a time when he was having an extramarital affair.

"It's not true," he said, insisting he had no knowledge of – or involvement in – how the spending had been billed or calculated.

Fred Thompson produced the surprise of the night with a YouTube video which was effectively an attack advertisement aimed at Mr Romney's record on abortion and Mr Huckabee's position on taxation.

The former senator, whose performance has been criticised as lacklustre and lazy in recent months, was significantly less aggressive, however, when it came to tackling his rivals face-to-face.

He intervened briefly in the immigration argument between Mr Giuliani and Mr Romney, saying: "We’ve all had people, probably, that we have hired, that in retrospect probably was a bad decision.”

This was an allusion to the former New York mayor's appointment of police commissioner Bernie Kerik - since indicted for fraud - but the reference was so obscure that it appeared to be lost on most of the audience.

Mr Huckabee, by contrast, entered the debate buoyed by opinion polls indicating that his poll surge was being replicated in other states beyond Iowa, including Florida, and he ended it knowing that he had delivered probably the most assured performance of the night.

He sidestepped a question over whether Christ would authorise the use of the death penalty, by saying: "Jesus was too smart to ever run for public office." Asked about his plan to abolish federal income tax, he was cheered as he said: "Most people in this country are more afraid of a [tax] audit than a mugging."

Even as he backed an expansion of the space programme – an important issue in Florida, where this debate was held – Mr Huckabee deftly used his brand of homespun, if slightly corny, humour to make his point, saying: "Maybe Hillary [Clinton] could be on the first rocket to Mars."

The Democratic frontrunner's presence was felt in other ways at the CNN/YouTube debate. The question that wrong-footed Mr Romney about letting gays serve openly in the military was submitted by retired Brigadier General Keith Kerr, who was present in the audience and given the opportunity of making a long rebuttal of the Republican candidates' answers. It later transpired that he was a member of Mrs Clinton's Lesbian, Gay, Bisexual and Transexual Americans Steering Committee.

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