Romney's aggressive demeanor over the past few days and his rivals' willingness to openly assail him reflect the stakes in the wide-open race for the Republican presidential nomination. Huckabee, the former Arkansas governor, defeated Romney in the Iowa caucuses on Thursday with an underfunded campaign. Now Romney faces a strong challenge from a resurgent McCain in New Hampshire's first-in-the-nation primary Tuesday. Michigan, where Romney was reared and is competing strongly, votes a week later.

Former Tennessee Sen. Fred Thompson, ex-New York Mayor Rudy Giuliani and Texas Rep. Ron Paul also participated in the 90-minute debate.

Romney walked onstage fresh from a victory earlier in the day in the scarcely contested Wyoming caucuses. Seeking to become the first Mormon president, he said the outcome was "just the beginning."

Two new polls in New Hampshire suggested McCain's momentum had carried him into a narrow lead over Romney, with Huckabee in third place. The surveys also suggested Huckabee had not yet profited from his victory in Iowa. But the results of an election in one state often take several days to show up in surveys in another state.

Romney, who has contributed more than $17 million of his own money to his bid, could hang in the race far longer than others. But it would be difficult, though not impossible, to rebound from a second straight loss, given that his strategy has been based on using consecutive victories to steamroll the field in later-voting states.

His rivals keyed in on Romney's record of reversing and shifting positions on issues such as abortion, gun control and gay rights as well as his recent attempt to cast himself as someone who can makes major changes and fix a dysfunctional Washington.

"We disagree on a lot of issues, but I agree you are the candidate of change," McCain said with a laugh, using Romney's latest theme against him.

During a discussion on Iraq, Romney told Huckabee, "Don't try and characterize my position."

"Which one?" Huckabee fired back.

Giuliani, too, slapped at Romney. Giuliani cited Ronald Reagan's granting amnesty to illegal immigrants in the 1980s and said: "I think he'd be in one of Mitt's negative commercials."

The criticism led Romney, who has run arguably the most negative campaign of any Republican, to plead several times for his rivals to refrain from personal attacks.

"You know, we're wise to talk about policies and not to make personal attacks," Romney said. At another point, he complained, "The continued personal barbs are interesting but unnecessary."

Yet, as the candidate begged for civility, his aides were busy e-mailing news releases that went after McCain personally, pointing out instances in which the senator used expletives against his political foes.

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Editor's note — Liz Sidoti covers presidential politics for The Associated Press.