Republican presidential hopefuls, led by frontrunner Mitt Romney, launched a final frenetic campaign push in New Hampshire Monday on the eve of the state's primary.

Polls showed the former Massachusetts governor, who won Iowa's caucus by a whisker last week, slipping slightly but still holding a commanding lead and looking to notch a second victory before the fight shifts to South Carolina.

Romney's rivals hoped for an shocking upset in New Hampshire's bellwether nominating primary -- where he lost in 2008 to the eventual nominee, Senator John McCain -- while they planned for an all-out assault on his record before the southern state's January 21 vote.

"I'm not worried. I'm happy," Romney replied at a rally in Exeter when asked whether he feared his 15-point edge over his nearest rival, Texas congressman Ron Paul, might lead overconfident supporters to stay home on Tuesday.

Paul, a small-government champion whose opposition to overseas entanglements breaks with Republican orthodoxy, pumped up his passionate base of mostly young voters in Meredith by dismissing his competitors as agents of "the status quo."

"They don't challenge the establishment," said the lawmaker, who trails Romney in the polls by a 35 percent to 20 percent margin. "I think the country is looking for something completely different."

A survey by Suffolk University in nearby Boston found support for former US envoy to China Jon Huntsman has increased to 11 percent, and former House speaker Newt Gingrich was at nine percent.

Gingrich, savaged in what he has denounced as false attack ads by Romney allies, declared that turnabout was fair play and said he and an independent Super-PAC (political action committee) backing him would soon be taking the fight to the frontrunner.

"This is a tough business, and should be, you're trying to pick someone strong enough to be president of the United States, it's just got to be factually correct," he told reporters at a town hall meeting in a Mexican restaurant.

The opinion poll found Christian conservative Rick Santorum had fallen to eight percent -- a slide pollsters have blamed on his virulent criticisms of gay rights -- while Texas Governor Rick Perry was at one percent.

In the second of a pair of debates held barely 10 hours apart, Santorum Sunday worked to soften an angry image shaped by his past comparisons of homosexuality to bestiality and by his fierce opposition to gay marriage and gay adoptions.

Asked what he would do if he had a son tell him he was gay, the former senator from Pennsylvania replied: "I would love him as much as I did the second before he said it. And I would try to do everything I can to be as good a father to him."

While Romney's rivals escalated their attacks on him, notably in Sunday's debate, he committed no major errors, making it unlikely that the assault would derail his better-funded, better-organized campaign.

The debates could shape Tuesday's vote, which may drive one or more candidates from the race, resetting a field that has been led alternately by Romney and successive conservatives who have surged and fallen back.

A vast campaign war chest and high-profile endorsements have fed Romney's image as the man to beat, but he faces stubborn doubts about his conservative credentials and has been unable to push his support from Republicans nationwide above 30 percent.

South Carolina is the first contest for the Republican nomination in the US South, where Romney faces pushback from conservatives who think he is a moderate flip-flopper and from evangelical Christians wary of his Mormon faith.

Leaving nothing to chance, Romney -- who secured McCain's endorsement last week -- campaigned late Sunday with rising Republican star Governor Chris Christie of New Jersey and home-state favorite Senator Kelly Ayotte.

The Romney campaign aimed to make 150,000 volunteer phone calls from the weekend through election night. One thousand volunteers -- half of whom came in from other states -- were fanning out throughout the state to knock on 10,000 doors. Already, 25,000 Romney yard signs were up.

Huntsman, who has basically bet his campaign on his showing in New Hampshire, scored one of the biggest applause-getting lines of Sunday's debate when he rebuked Romney for attacking his service as President Barack Obama's first ambassador to China.

The former diplomat said he was "putting my country first," above politics.

Romney retaliated, saying "the person who should represent our party running against President Obama is not someone who called him a remarkable leader and went to be his ambassador in China."

But Huntsman won more applause when he shot back that "this nation is divided ... because of attitudes like that. The American people are tired of the partisan division. They have had enough."