An assault on Mitt Romney’s business career intensified Monday after the front-runner for the Republican presidential nomination made an ­off-the-cuff comment that his opponents say shows he was a corporate predator who sought profits at the expense of workers.

At a breakfast of business leaders in Nashua, N.H., Romney said, “I like being able to fire people who provide services to me.” The former Massachusetts governor was referring to health insurers that don’t provide adequate care, but on the eve of the New Hampshire primary, his opponents nevertheless feasted upon the apparently unscripted remark by a habitually cautious candidate.

“Governor Romney enjoys firing people; I enjoy creating jobs,” former Utah governor Jon Huntsman, who polls suggest is enjoying a late surge here, told reporters in Concord. “It may be that he’s slightly out of touch with the economic reality playing out in America right now, and that’s a dangerous place to be.”

Former House speaker Newt Gingrich (Ga.) went further, criticizing the type of business Romney engaged in. “Look, I’m for capitalism,” Gingrich said on NBC’s “Today” show. “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.”

Instead of sprinting to the finish before Tuesday’s primary, which he is heavily favored to win, Romney spent his final day on the New Hampshire campaign trail explaining and defending his role as co-founder and chief executive of Bain Capital. The venture capital firm invested in start-ups such as Staples, an office supplies superstore, but also oversaw large-scale job losses through leveraged buyouts and restructuring.

“Free enterprise will be on trial,” Romney told reporters in Hudson. “I thought it was going to came from the president, from the Democrats on the left, but instead it’s coming from Speaker Gingrich and apparently others. And that’s just part of the process. I’m not worried about that. I’ve got broad shoulders.”

Romney came under siege in a debate Sunday over his work at Bain, and his comment Monday seemed to give his opponents an opening to try to turn his greatest asset — that he would be uniquely skilled at creating jobs and turning around the nation’s economy — into a liability. It also provided evidence for those trying to cast Romney as out of touch with the struggles of working Americans.

The attacks over Romney’s business background come as surveys showed his healthy lead in New Hampshire slipping. A Suffolk University-7 News tracking poll Monday showed him falling to 33 percent, from 43 percent last week, with four other candidates — Huntsman, Gingrich, Rep. Ron Paul (Tex.) and former senator Rick Santorum (Pa.) — in a volatile race for second.

The battle over Bain is certain to intensify as the race moves to South Carolina, where independent groups that support Romney and Gingrich are planning multimillion-dollar TV ad blitzes.

It’s a fight that Gingrich, in particular, is eager to have.

“It’s a legitimate question about exactly what happened: Where did the money go? Who got the money? What happened to the people involved?” he told reporters. “He’s the one who went around and said he has 20 years’ experience — fine. Now let’s talk about the 20 years’ experience.”

Texas Gov. Rick Perry, campaigning in South Carolina, accused Romney of having “looted” a South Carolina firm, getting “rich off failures and sticking it to someone else.”

Romney waved away the intensifying criticism and tried to pro­ject confidence heading into Tuesday’s primary. He joked that he hopes to double the eight-vote margin he had in the Iowa caucuses. “I don’t think I could handle another night like that,” he said.

Addressing reporters after a campaign stop at a metal-fabricating plant, Romney said his opponents took out of context his comment about firing people.

“I’m happy to describe my experience in the private economy,” he said. “I understand that in politics people are going to try and grasp at anything, take it out of context and make it something it’s not.”

Romney made the comment in response to a question about health policy. He tried to use the breakfast forum sponsored by the Nashua Chamber of Commerce to defend his career.

He began his speech to more than 300 area business leaders by saying that he “started off, actually, at the entry level.”

“I’m not perfect by any means,” Romney said, adding: “If I’m president, I will take my 25 years of experience in the private sector and use that to rekindle the basis of our economic foundation.”

Not all of Romney’s rivals criticized his years at Bain. Santorum told reporters: “I’m not making it a liability. I believe in the private sector.”

Hours later, visiting an American Legion hall in Somersworth, Santorum jabbed Romney for other reasons.

“Are you going to send a message that the guy who’s spent the most money, the most time, who’s run the most times here in New Hampshire, that’s the guy you want, because he’s been around? Because it’s his turn?” Santorum asked. “Ladies and gentlemen, we win elections when our people are excited about who to vote for.”

President Obama’s reelection strategists have long planned to attack Romney on his Bain career if he makes it to the general election, but the issue until now had stayed largely off the table during the Republican primary.

Romney’s advisers said they welcomed a debate over his business record with the other Republican candidates. “If they want to fight on this issue down there, how capitalism works, good,” said Tom Rath, a senior Romney adviser. “Come into my parlor, said the spider to the fly.”

Winning Our Future, a pro-Gingrich group backed by a $5 million donation from casino mogul Sheldon Adelson, is planning a $3.4 million TV and radio ad campaign in South Carolina. The group plans to air parts of a movie about Bain Capital, featuring interviews with people who attribute job losses to the company. Meanwhile, Restore Our Future, a pro-Romney super PAC, revealed plans for a $2.3 million ad blitz of its own.

Still, Romney was on the defensive for the second day in a row, with him and his aides scrambling to explain not just his remark about firing people but also his reference to having feared receiving a “pink slip” during his business career.

At a rally Sunday in Rochester, N.H., Romney said: “I know what it’s like to worry whether you’re going to get fired. There were a couple of times I wondered whether I was going to get a pink slip.”

By Monday morning, Perry was mocking Romney. “I have no doubt Mitt Romney was worried about pink slips — whether he was going to have enough of them to hand out,” Perry told a breakfast gathering in South Carolina. He labeled Romney “one of the wealthiest men, I suppose, who has ever run for the presidency of the United States, the son of a multimillionaire.”

Romney, asked to offer a specific example of when he feared losing his job, spoke only in generalities.

“I got an entry-level position like the other people that were freshly minted MBAs,” Romney said. “Like anybody that starts at the bottom of an enterprise, you wonder, when you don’t do so well, whether you’re going to be able to hang on to your job, and you wonder if the enterprise gets in trouble, you know, will you be one of those that’s laid off.”

Staff writers Dan Balz, Amy Gardner and Rosalind S. Helderman in New Hampshire contributed to this report.