Reuters) - Republican presidential candidates stepped up their attacks on Mitt Romney in a televised debate on Sunday and put the usually unflappable front-runner on the defensive just two days before voters in New Hampshire head to the polls.

Rick Santorum, a social conservative whose campaign caught fire in Iowa and who has pinned hopes more on the next contest in South Carolina, took aim at Romney's political past and emerged unscathed from an exchange about gay rights.

Criticism of Romney zeroed in on the perception that the former Massachusetts governor would be the strongest candidate against Democratic President Barack Obama in the November election.

Wasting no time, former House of Representatives Speaker Newt Gingrich said Romney would "have a very hard time getting elected."

"There's a huge difference between a Reagan conservative and somebody who comes out of the Massachusetts culture who essentially has a moderate record," Gingrich said during the NBC/Facebook debate in Concord, New Hampshire.

Gingrich's popularity in Iowa cratered in the face of millions of dollars of attack ads run from a Super PAC, an outside group that was supporting Romney.

Gingrich took Romney to task again about the ads on Sunday, saying the spots contained multiple lies. Romney distanced himself from the ads.

"Speaker, hold on a second, I can't direct their ads. If there's anything that is wrong, I hope they take it out," said Romney of his SuperPAC supporters.

The two sparred over the negative campaigning each accused each other of engaging in. "I wouldn't call you some of the things you've called me, I just think that's over the top," Romney said of Gingrich.

Opinion polls show Romney holds a wide lead in New Hampshire, which holds its first in the nation primary election on Tuesday, and also leads in South Carolina, the next state in the nominating process.

"If his record was so great as governor of Massachusetts, why didn't he run for re-election," Santorum, a former senator from Pennsylvania, said of Romney, who launched his unsuccessful 2008 White House bid just weeks after leaving the statehouse after one term.

Santorum has been riding a wave of popularity after a narrow second-place finish to Romney in the first Republican presidential nominating contest in Iowa last week.

Often in the news for negative comments about gays, Santorum managed to strike a conciliatory tone, saying the "every person in American gay or straight treated with respect" and that he would still love a gay son.

SOLID CONSERVATIVE

Romney defended himself as "a solid conservative" who was in politics as a detour from his business career as a venture capitalist, and kept his focus more on Obama than on his Republican rivals.

"I happen to believe that if we want to replace a lifetime politician like Barack Obama ... we've got to choose someone who is not a lifetime politician, who has not spent his entire career in Washington."

Gingrich bristled at Romney's attempts to paint himself as a reluctant politician.

"Can we drop a little bit of the pious baloney?" he quipped. "You were running for president while you were governor."

One of the biggest applause lines came from Jon Huntsman, who responded, albeit belatedly, to a comment Romney had made about him in Saturday night's debate in Goffstown, New Hampshire.

Romney had slapped Huntsman for "implementing" Obama's agenda as U.S. ambassador to China, a post he held until April.

Addressing debate moderator David Gregory, Huntsman said: "This country is divided, David, because of attitudes like that. ... The American people are tired of partisan divisions."

The candidates, who are essentially jockeying for a strong second place finish in New Hampshire behind Romney to gain some momentum heading to the South Carolina primary on January 21, also attempted to take aim at each other.

When the moderator pointed out that Texas congressman Ron Paul has a thin legislative record despite a long career in Congress, Santorum found an opening.

"He's never really passed anything of importance. He's never been able to accomplish anything. He has no track record. He's been out there on the margins," Santorum said.

Texas Governor Rick Perry, an afterthought in New Hampshire, also targeted Obama, declaring the president a "socialist."