Mitt Romney faced blistering new attacks from his rivals for the Republican presidential nomination over jobs lost at firms bought by his hugely rich venture capital company.

On track to win New Hampshire's first-in-the-nation primary on Tuesday by a wide margin, the former Massachusetts governor found himself justifying what he has described as his greatest political strength: his business acumen.

"I have been in business. I have learned some things fail and some things succeed, that's how it works," he told the Nashua Chamber of Commerce. "Those that succeed make our overall nation stronger."

"Our system works. Free enterprise works," said Romney, who touts his leadership of private equity firm Bain Capital as his signal qualification to take on President Barack Obama in the November 6 elections.

Former House speaker Newt Gingrich, trailing in the polls after a blitz on his own record by Romney allies, led the charge against his rival over his time at Bain, which dismantled some of the firms it bought while reaping vast profits.

"They apparently looted the companies, left people totally unemployed, and walked off with millions of dollars," Gingrich told NBC television.

"Look, I'm for capitalism," he said. "But if somebody comes in, takes all the money out of your company, and then leaves you bankrupt while they go off with millions, that's not traditional capitalism."

The attacks came as Romney hoped to notch a commanding victory here after eking out a win in Iowa, and looked to carry South Carolina's January 21 primary -- a trifecta that could make him unstoppable.

The Democratic National Committee -- effectively the political arm of the White House -- also got in on the act, putting out a video snippet of Romney's Nashua remarks in which he boasts, "I like being able to fire people."

"The insurance company will have an incentive to keep you healthy. It also means that if you don't like what they do, you can fire them. I like being able to fire people that provide services to me," he said.

Another rival, former US China envoy Jon Huntsman, piled on, saying: "Governor Romney enjoys firing people, I enjoy creating jobs" and adding "it may be that he's slightly out of touch with the economic reality playing out in America right now."

Asked whether his remarks could feed that perception, Romney denounced those who "think that I should spend my entire campaign carefully choosing how everything I say relates to people, as opposed to saying my own experience and telling my own experience."

"That would make me a very different person than I am," he said, adding: "I'm going to tell you about myself. And if people like that, great."

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.

"South Carolina is not his kind of territory," Gingrich told Fox News.

But the Bain-related attacks effectively opened a new front against Romney, who has had to walk a difficult balance between boasting of his private sector success and avoid being tied to Wall Street, which is broadly unpopular.

And they could help Obama, who faces an uphill fight to reelection because of the sour US economy and high joblessness.

Stung, Romney told reporters at a hastily called press conference on the factory floor of Gilchrist Metal Fabricating after a rally with supporters that the criticisms meant "free enterprise will be on trial" on Tuesday.

Asked about the DNC attack, he replied that "in politics, people are going to try and grasp at anything, take it out of context, and make it something it's not."

"And by the way that's the nature of the process, I've got to be an adult about it and recognize that goes with the territory," said Romney, whose campaign earlier this year unapologetically ripped Obama's words about the economy out of context for a political ad.

A daily New Hampshire tracking poll from Suffolk University in nearby Boston found Romney leading the pack with 33 percent of likely voters, followed by Representative Ron Paul at 20 percent, Huntsman at 13, Gingrich at 11, and Christian conservative Rick Santorum at 10 percent.

Perry and former Louisiana governor Buddy Roemer trailed the pack in low single digits.