MANCHESTER, N.H. - Republican rivals have a chance to stop front-runner Mitt Romney's march to the presidential nomination in back-to-back weekend debates that could help define the contest just days before the first-in-the-nation New Hampshire primary.

In a race largely driven by 13 previous debates, Romney has emerged mostly unscathed by the six or seven opponents who are competing with him for the nomination to face President Barack Obama in the November election.

That could change with Saturday night's debate or the one scheduled Sunday morning, as rivals Newt Gingrich and Rick Santorum seek to slow Romney's momentum. In particular, Gingrich, the former speaker of the House of Representatives, is looking to keep his candidacy afloat while Santorum, a former U.S. senator from Pennsylvania, hopes to capitalize on his surprising neck-and-neck performance against Romney in Iowa's leadoff caucuses last Tuesday.

There are fewer than 12 hours between an ABC News/WMUR debate Saturday night and an NBC News/Facebook debate on Sunday morning's "Meet the Press."

Also sharing the debate stage will be libertarian-leaning Texas Rep. Ron Paul, the third-place finisher in Iowa; Texas Gov. Rick Perry, who briefly considered quitting the race after his fifth-place finish in Iowa; and former Utah Gov. Jon Huntsman, who served as Obama's ambassador to China. Huntsman skipped Iowa to focus exclusively on New Hampshire.

"Two debates! Onetomorrow night, one the next morning. Why even stop?" Romney declared Friday at a campaign stop in Tilton, New Hampshire. "Why don't we just go right through? It's nonstop!"

Debates can have unforeseen impact. Just days before the New Hampshire primary in 2008, Obama used that venue to call Democratic rival Hillary Rodham Clinton "likable enough" -- a dismissive comment that didn't sit well with her supporters, particularly women. Obama, who had a significant lead in polls, lost the New Hampshire primary.

Romney's rivals have a serious gap to close in New Hampshire -- and, recent polls show, in South Carolina, which holds the first primary in a southern state on Jan. 21.

Ordinarily, the week between Iowa and New Hampshire is one of the most intense of the entire president campaign. That hasn't been as true this year in New Hampshire, given Romney's home-field advantage in the northeastern state. He served four years as governor of next-door Massachusetts and owns a vacation home on a New Hampshire lake.

Romney's rivals have all but conceded he will win Tuesday's New Hampshire primary, but are looking to hold down his margin of victory and make him look vulnerable heading into South Carolina. Romney won the Iowa caucuses eight votes over r Santorum despite having big advantages in campaign funds and organization.

Romney would be difficult to stop if he manages to sweep the first three contests. A victory in South Carolina would be huge for him. It's a conservative state and Romney has struggled to win over Republicans who view him as too moderate. Romney underscored the state's importance by leaving New Hampshire to campaign in South Carolina on Thursday and Friday.

A TIME/CNN/ORC poll released Friday showed Romney soaring into the lead in South Carolina, with 37 per cent support among the state's Republican's, a 17-point gain since early December. Santorum was at 19 per cent, a 15-point surge. That put him in a statistical tie with former Gingrich, who had plummeted from 43 per cent support, despite his southern roots as a congresssman from neighbouring Georgia.

Romney, who has touted his business background, has cast himself as the candidate best-positioned to take on Obama, whose prospects for re-election have been hurt by the United States' slow recovery from a recession.

But Obama got a boost Friday when the government reported the creation of 200,000 new jobs in December and the unemployment rate dropping to 8.5 per cent, its lowest level in almost three years.

Romney has also benefited because conservatives have been unable to rally around an anti-Romney alternative. Texas Rep. Ron Paul has drawn support from the party's libertarian wing. Santorum, Gingrich and Texas Gov. Rick Perry have all drawn votes from social conservatives wary of Romney's shifting positions on abortion and gay rights. This weekend, a group of prominent Christian conservative leaders are meeting in Texas to decide if they should throw their backing behind a single candidate.

So far, Romney's rivals have been looking past New Hampshire. Santorum, the most outspoken religious conservative in the field, has campaigned in the state but has been peppered with hostile questions about his opposition to gay marriage and comments about homosexuality.

Gingrich has been talking of merely holding Romney's winning total under 50 per cent in New Hampshire while previewing his expected lines of attack against Romney as a "Massachusetts moderate" in the weekend debates. In early December, Gingrich was leading or tied with Romney in polls of Iowa Republicans, but his support plummeted after Romney's supporters unleashed a wave of negative TV ads targeting Gingrich.

Gingrich, who has made his mark during debates, has aggressively criticized Romney in recent days. He called Romney a "liar" and also said President Obama would laugh at Romney if he were the nominee. He said the health care reform plan introduced by Romney in Massachusetts funded coverage for abortions and also included a mandate to purchase insurance which Obama adopted for his national health care law that Republicans loathe.

Santorum has also attacked Romney in recent days.

"The only way Republicans lose is if we screw this up and nominate another moderate who has taken multiple positions on every major issue of our time," he wrote supporters in a fundraising appeal. Santorum has little time to try to convert his near-victory in Iowa into a campaign organization in New Hampshire and other early voting states.

Paul, who has been running a distant second to Romney in New Hampshire polls, has focused his criticism on Santorum.

"He brags about being for a balanced budget amendment but never did anything about it," Paul said of Santorum's time in the Senate. "He voted four or five times to raise the debt ceiling. He voted to double the size of the Department of Education."

New Hampshire Secretary of State Bill Gardner says he expects 250,000 ballots to be cast in Tuesday's highly contested Republican presidential primary.