CONCORD, N.H. - Fellow Republican candidates hacked away at Mitt Romney's front-runner status Sunday in the second debate in two days as New Hampshire voters were deciding on their choice for the party's nomination to challenge President Barack Obama in November.

Former speaker of the House of Representatives Newt Gingrich abandoned his vow to run a positive campaign, issuing sharp attacks on Romney and, at one point, accusing the former Massachusetts governor of running a campaign of "pious baloney" for saying he's not a career politician. Gingrich also accused Romney of hiding behind inaccurate attack television ads by his allies.

Like Gingrich, former Pennsylvania Sen. Rick Santorum hit Romney hard for his moderate record, one that conservative Republicans say makes him a weak Republican standard-bearer in a time of deep and bitter partisan divisions across the country.

The six candidates debated in a gathering in Concord, New Hampshire, sponsored by the NBC television network and Facebook.

Voters in New Hampshire, the tiny Northeastern state that holds the nation's first primary election of the nominating season, will cast their ballots Tuesday for a favourite from among the Republicans still in the race.

Romney is the overwhelming favourite in the state, where he maintains a vacation home and is well know to voters in New Hampshire for his four-year term as governor in neighbouring Massachusetts. Romney's rivals hope to hold down his margin of victory and are jostling to finish in the top tier of candidates to gain momentum heading into the next nominating contests in southern states with large numbers of social conservatives who may be wary of Romney's shifting positions on abortion and gay rights.

The issue of which of the candidates is most electable has become a major issue in the contest for the Republican nomination. Obama is vulnerable as he seeks a second term because of the weak U.S. economy, continuing high unemployment and the slow recovery from the 2007-2009 Great Recession and near financial meltdown in the last months of the administration of former President George W. Bush.

Former Utah Gov. Jon Huntsman, who served as Obama first ambassador to China, provided perhaps the strongest retort of the debate to Romney. The front-runner said in Saturday night's debate that Huntsman was not qualified to be the Republican nominee because he had served in the Obama administration.

"I just want to remind the people here in New Hampshire and throughout the United States, he criticized me while he was out raising money for serving my country in China, yes, under a Democrat, like my two sons are doing in the United States Navy," Huntsman said. "They're not asking what political affiliation the president is."

When Romney again criticized Huntsman for accepting the ambassadorship from Obama, Huntsman shot back: "This nation is divided ... because of attitudes like that."

Huntsman has made New Hampshire the near-total focus of his bid for the nomination, and late polling by Suffolk University suggests he is gaining momentum while Santorum is fading. Huntsman did not campaign in Iowa, whose caucuses last Tuesday led off the state-by-state nominating contests.

New Hampshire is more fertile territory for Huntsman because its primary is open to self-declared independent voters. Iowa's caucuses are dominated by more conservative Republicans, including many evangelicals.

Romney won Iowa by the slimmest of margins, eight votes over Santorum. Texas Rep. Ron Paul, a strict small-government libertarian, was third in the Iowa caucuses. Rep. Michele Bachmann, who finished sixth in Iowa, dropped out of the race after have been Romney's top challenger last summer.

Texas Gov. Rick Perry also finished badly in Iowa, but has continued campaigning, primary now in South Carolina. After the New Hampshire vote, the selection process moves to the Jan. 21 primary in that extremely conservative southern U.S. state.

All 50 U.S. states engage in choosing the nominee through caucuses or primary elections that continue until June. The nominee is named in August at the Republican National Convention in Tampa, Florida.

Generally, Sunday's debate adhered to the trend of the one the night before but was more lively for the pointed shots the candidates took at Romney.

He largely, as he had Saturday night, shrugged off the attacks and worked to turn the focus onto Obama and the long road to economic recovery. Obama has been "anti-investment, anti-jobs and anti-business," Romney said.

Santorum punched hard at Romney, asking why he hadn't sought re-election as governor after one term in Massachusetts.

Moments later, Gingrich appeared irked and accused Romney of using more than his allotted time to respond.

"Could we drop a little bit of the pious baloney. The fact is you ran in '94 and lost (to Ted Kennedy in the Massachusetts Senate race). ... You were running for president while you were governor. ... You've been running consistently for years."

Romney denied the accusation briskly. "Politics is not my career," he said. "My life's passion has been my family, my faith, my country."

Gingrich also challenged Romney's conservative credentials, calling him a "relatively timid Massachusetts moderate" whose state ranked fourth from the bottom in job creation when he was governor.

Obama's camp weighed in against Romney even as the debate was in progress.

The president's chief campaign adviser David Axelrod said conservative and moderate Republicans are having problems trusting Romney, saying it's clear the party front-runner faces resistance from a majority of Republican voters. Romney's support is under 30 per cent in most national surveys and some early voting states.

Axelrod also took aim at Romney's work at the Bain Capital private equity company. Romney has never documented his claim that he was a creator of more than 100,000 jobs while leading Bain. Critics cite jobs lost at Bain-acquired or Bain-supported firms that closed, cut back or shifted employment overseas.

Axelrod told ABC's "This Week" that Romney is "a corporate raider" and that "bringing a Bain mentality" to running the economy undercuts Romney's candidacy.

That will likely be the crux of the Obama message if Romney become the nominee.

Gingrich also raised Romney's record at Bain in the debates, referring to published accounts that described how some workers were laid off after Bain invested in their companies.