Mitt Romney’s opponents, seizing upon what could be one of their last opportunities to blunt his accelerating ­momentum toward the GOP presidential nomination, trained their fire on the front-runner Sunday.

Romney’s opponents took the parts of his résumé that he touts as strengths and offered them as evidence that he lacks authenticity, conviction and consistency. The sparring began on a debate stage and continued as the candidates fanned out across the state in their last weekend of campaigning.

In the latest polls, Romney has been maintaining a lead of more than 20 points over his rivals. If that kind of margin holds through Tuesday, it could provide Romney a burst of momentum that might make him all but unstoppable in South Carolina and Florida later this month.

But New Hampshire is known for its independence, and it has a history of tripping up leading candidates with last-minute surprises on primary day.

Romney’s rivals, hoping that something like that will happen this time, set fierce new terms of engagement almost from the opening moments of their debate Sunday morning.

“Can we drop a little bit of the pious baloney?” former House speaker Newt Gingrich asked Romney, after the former Massachusetts governor once again portrayed himself as a career businessman with a disdain for lifelong politicians.

“The fact is, you ran in ’94 and lost. That’s why you weren’t serving in the Senate,” Gingrich said. “You had a very bad reelection rating [as governor]. You dropped out of office. . . . You were running for president while you were governor.

“You have been running consistently for years and years and years,” Gingrich added. “So this idea that suddenly citizenship showed up in your mind, just level with the American people.”

When Romney promoted his record cutting taxes and balancing budgets as governor, former senator Rick Santorum (Pa.), who fought him to a virtual tie in last week’s Iowa caucuses, retorted: “If you didn’t want to even stand before the people of Massachusetts and run on your record, if it was that great, why did you bail out?”

And where Romney had cited his 1994 Senate bid against Edward M. Kennedy as a heroic and quixotic challenge to “the policies of the liberal welfare state,” Santorum said Romney’s loss resulted from a lack of spine.

“He wouldn’t stand for conservative principles,” Santorum said. “He ran from Ronald Reagan. And he said he was going to be to the left of Ted Kennedy on gay rights, on abortion, a whole host of other issues.”

Next up was former Utah governor Jon Huntsman. Defending himself against Romney’s barbs about having served as President Obama’s ambassador to China, Huntsman suggested that Romney puts partisanship above country.

“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, in an oblique reminder that none of Romney’s five sons have been in the military. “They’re not asking who — what political affiliation the president is. I want to be very clear with the people here in New Hampshire and this country: I will always put my country first.”

The unusual morning forum was the 15th debate of the campaign season — and the second in 10 hours — for the candidates. It was sponsored by NBC’s “Meet the Press,” the New Hampshire Union Leader and Facebook.

In the previous night’s event, which aired on ABC, Romney’s opponents had landed few blows on the front-runner.

Defending his record

Romney, initially rattled under Sunday morning’s barrage, tried to defend himself — but at times seemed to fuel some of the most damaging perceptions about him.

For instance, he reminded viewers of his background of wealth and privilege when he recounted a bit of “good advice” that he got from his father, George Romney, a wealthy auto executive who served as governor of Michigan.

“He said, ‘Mitt, never get involved in politics if you have to win an election to pay a mortgage,’ ” Romney recalled.

Later, at a town hall meeting in Manchester with Hispanic supporters at a Mexican restaurant, Gingrich blasted what he said was Romney’s suggestion that only the wealthy should run for office. “We want everyday, normal people to be able to run for office, not just millionaires,” he said.

At a midday rally in Rochester, meanwhile, Romney defended his career in private equity against charges by Gingrich’s allies that he had acted as a corporate predator.

“I spent my career in the private sector. I’m not perfect, but I do get it, and I will use what I know to get America to work,” he said. Romney also said there had been times in his life when he had been worried about getting laid off, though neither he nor his campaign provided specifics.

Romney was not the only target.

Ron Paul, who is running second to Romney in most New Hampshire polls, came under fire from Santorum, who said Paul’s political appeal stems from an economic proposal “that he’s never been able to accomplish.” But, Santorum said, Paul could bring troops home, as he has proposed doing, the moment he arrives at the White House.

“The problem with Congressman Paul is, all the things that Republicans like about him he can’t accomplish and all the things they’re worried about, he’ll do Day One,” Santorum said.

“We can’t stay in 130 countries, get involved in nation-building,” Paul replied. “We cannot have 900 bases overseas. We have to change policy.”

All of the candidates seemed on top of their games, and the months of practice they have had in honing their arguments showed.

Some do-overs

Two took a chance for do-overs on earlier muffed moments.

Gingrich had incurred the wrath of the Republican establishment in the spring — and nearly torpedoed his campaign just a few days after launching it — by calling a Medicare overhaul proposed by Rep. Paul Ryan (R-Wis.) “right-wing social engineering.” But on Sunday, he said that the latest version of the proposal “actually incorporates allowing people to choose and allows them to stay in traditional Medicare with the premium support model or go to new methods. And I think it’s a substantial improvement, because it allows for a transition in Medicare in a way that makes sense.”

And Texas Gov. Rick Perry made a wry, self-deprecatory reference to his own struggles in the debates, which have reduced his standing from front-runner to afterthought in the race.

Seizing on a question asked of someone else, Perry described the “three areas” of government he would cut, finally getting right the answer he famously flubbed on a debate stage in Michigan last year.

“It would be those bureaucrats at the Department of Commerce and Energy and Education that we’re going to do away with,” Perry said, grinning and turning to his rivals to hold up three fingers.

Negative advertising

The debate ended as it began: with a squabble, this time between Romney and Gingrich over the tenor of ads that a super PAC supporting Romney has run against Gingrich.

Gingrich repeated his view that the ads are false and that Romney bears responsibility for the onslaught despite the fact that the PAC is independent from the Romney campaign.

“Governor, I wish you would calmly and directly state it is your former staff running the PAC,” Gingrich said. “It is your millionaire friends giving to the PAC. And you know some of the ads are — aren’t true. Just say that. It’s straightforward.”

Romney responded, “I haven’t seen them.” But he went on to recite a long list of claims made in the ads, all of which, he said, were true.

“The ad I saw said that you’d been forced out of the speakership. That was correct,” he said. “It said that you had sat down with Nancy Pelosi and argued for a climate-change bill. That was correct.”

After the debate, Romney spokesman Eric Fehrnstrom explained the apparent inconsistency this way: “He hasn’t seen all of the super-PAC ads, but there was one in particular which he saw that he described on the debate stage.”

Staff writers Aaron Blake, Rosalind S. Helderman and Philip Rucker contributed to this report.