THE knives will come out at back-to-back debates this weekend as US Republican presidential hopefuls frantically jockey for position days before New Hampshire's key primary.

Debates are once again the main show in the 2012 race after candidates spent two weeks on the road campaigning in coffee shops and pizza places through Iowa and the New England state.

The six contenders go at it twice within 12 hours; the first was last night, then again today. It is their last, best chance to sway large numbers of voters, with New Hampshire set to vote on Tuesday.

Campaigning in New Hampshire has been reasonably muted as the teams prepare their candidates for the TV showdowns on ABC and then NBC's Meet The Press.

"The debates are their opportunity to be seen by tens of thousands of voters," said former state Republican Party chairman Fergus Cullen. "They are critically important."

More than a dozen debates thus far have led to defining moments in the race to find a Republican challenger for the presidency.

Rick Perry's "oops" moment when he could not list all three government agencies he wanted to eliminate contributed to his poll collapse. Mitt Romney's offer of a $10,000 bet with Perry made him look like an out-of-touch high-flier.

This weekend's debates may breathe new life into Newt Gingrich, who has fallen apart under the weight of negative ads and attacks in recent weeks. And former Pennsylvania senator Rick Santorum, who has complained about not being asked enough questions at debates, will now literally be at centre stage and may have to face tough questions about his Senate record and his anti-gay rights stance.

Romney's objective at the sessions is to simply hold his own and not make any mistakes.

He has a big lead in New Hampshire after winning the Iowa caucuses last Tuesday by a mere eight votes. But at a dinner on Friday night, he said: "Let me tell you, those polls, they can just disappear overnight." And he urged supporters to "give me a bigger margin than eight votes if you possibly can".

A big win in New Hampshire and then a victory in the next vote in South Carolina would all but give him a stranglehold on the Republican presidential nomination.

The television debates may represent a do-or-die chance for former US ambassador to China, Jon Huntsman, who has been trailing badly in the polls despite skipping Iowa to focus on New Hampshire.

Santorum has Romney in his sights after narrowly losing the Iowa caucuses. He wants to jump into second place in New Hampshire and emerge as the main conservative alternative to Romney looking ahead to January 21 in South Carolina, where his social conservative message has a better chance.

Ron Paul – currently second behind Romney in New Hampshire – may get some heat over a TV advert from a group called "NHLiberty4Paul" that targeted Huntsman and his adopted daughters. One is from China, the other from India.

"American values. Or Chinese," the ad asks, to a soundtrack of Chinese music. It calls Huntsman "the Manchurian Candidate" and ends with an image of him as China's Communist leader Mao Zedong, and the words "Vote Ron Paul".