The knives will come out at back-to-back debates this weekend as 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, first in Iowa and now in New Hampshire.

The six contenders will go at it twice within 12 hours, first on Saturday night and then on Sunday morning. It is their last, best chance to sway large numbers of voters with New Hampshire set to vote on Tuesday.

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 to Democratic President Barack Obama.

Texas Gov. Rick Perry's Oops moment when he could not list all three government agencies he wanted to eliminate contributed to his collapse in polls. 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, the former speaker of the U.S. House of Representatives who has fallen apart under the weight of negative ads and attacks in the weeks since he built a lead in the polls based on strong debate performances.

Former U.S. Sen. Rick Santorum of Pennsylvania, who often complained about not being asked enough questions at debates, will now literally be at center stage and may have to face tough questions about his record in the Senate and his anti-gay rights stance.

The rest of the pack could turn against former Massachusetts Gov. Mitt Romney, the front-runner by far, whose objective at the twin sessions is to simply hold his own and not make any mistakes. He urged his supporters not to get complacent.

Romney is sailing along with a big lead in New Hampshire after winning the Iowa caucuses last Tuesday by a mere eight votes.

Let me tell you, those polls, they can just disappear overnight, Romney said at a spaghetti dinner in Laconia on Friday night, urging supporters to give me a bigger margin than eight votes if you possibly can.

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

Romney a Target

Romney is likely to keep his focus on Obama, and try to offer a forward-looking message on how to create jobs.

He and his team understand there will be a lot of attacks coming at him, said a Romney backer, Republican strategist Phil Musser. I expect he'll be prepared for that.

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

The debate may represent a do-or-die chance for former U.S. 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 Jan. 21 in South Carolina, where his social-conservative message has a better chance of being heard.

Santorum goes into the debates suggesting Romney's health-care plan that he developed as governor of neighboring Massachusetts meant that Romney is not different enough from Obama.

That's why New Hampshire can't faint, he said in Dublin on Friday. They have to stand and be bold. We need a clear contrast, someone who paints a very different vision.

Gingrich has been rampaging through New Hampshire calling Romney a timid Massachusetts moderate. Gingrich was left embittered by his fourth-place finish in Iowa, blaming attacks from Romney and his backers.

He even raised taxes on people who were blind, Gingrich said of Romney at a tele-town hall on Friday, referring to an attempt in Massachusetts to impose a $10 fee for receiving a certificate of blindness.

Ron Paul, who is running second behind Romney in New Hampshire, may get some heat over an ad from an outside group called NHLiberty4Paul that targeted Huntsman and his adopted daughters: One is from China, the other is 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 Huntsman dressed as China's Communist leader Mao Zedong, and the words Vote Ron Paul.

Paul told CNN he disavowed the ad and could not control the actions of all his supporters. Huntsman said he did not mind ads targeting him, but that attacking his daughters was out of bounds.

(Additional reporting by Sam Youngman, Jason McLure and Ros Krasny; Editing by Sandra Maler)