They may have stood apart on economic issues, but the candidates were united in opposition when asked about gay marriage, which is legal in New Hampshire and Iowa.
"There's every right for people in this country to form long-term relationships with each other, that doesn't mean they need to call it marriage," said Romney.
"It's a huge jump from being understanding, considerate, concerned [for same-sex couples], which we should be," Gingrich said, "to saying we're therefore going to institute the sacrament of marriage as though it has no basis. The sacrament of marriage is based on a man and a woman, has been for 3,000 years, is at the core of our civilization and is worth protecting and upholding."
"I think protecting and upholding that doesn't mean you need to make life miserable for others," he added.
"I believe the issue of marriage itself is a federal issue," said Santorum, a longtime critic of same-sex marriage. "Marriage is... a foundational institution in our country and we have to have a singular law with respect to that."
Then Gingrich trained fire on the news media for not adequately reporting on recent decisions by Catholic Charities groups to cease adoption services in several states which would have required them to work with same-sex couples.
"You don't hear the opposite question asked: should the Catholic Church be forced to close its adoption services in Massachusetts because it won't adopt to same-sex couples?"
"The bigotry question goes both ways. And there's a lot more anti-Christian bigotry today than there is on the other side and none of it gets covered by the media," he said, drawing loud applause.
"People in this room think Speaker Gingrich is right, and I do too," interjected Romney.
During the debate several of the candidates, including Romney and Gingrich, urged party unity, saying they'd support anyone on the stage if he were to become the GOP nominee to challenge Obama. But not Paul, who some speculate could mount a third party run.
When asked about the possibility of making an independent general election bid, Paul insisted he has "no plans to do it" but would not definitively rule it out.
"I don't intend to do it. And somebody pushed me a little bit hard and said why don't you plan to do it? I just -- I don't want to. So I have no intention," he said. "But I don't know why a person can't reserve a judgment and see how things turn out? You know, in many ways I see the other candidates as very honorable people, but I sometimes disagree with their approach to government."
Jon Huntsman, who has focused the bulk of his campaign efforts in New Hampshire, struggled to maintain his ground and came under a late, last-minute surprising attack by the frontrunner himself.
Romney attacked Huntsman's work as ambassador to China under the Obama administration, saying that as president, he wouldn't talk about China.
"I'm going to tell the Chinese, it's time to stop. I'm not going to let you kill American jobs," he said.
Huntsman responded in Chinese, saying, "he doesn't understand the situation."
Romney, the clear leader in the debate, has never looked stronger than he does right now. He is well ahead of his rivals in the Granite state, garnering 44 percent of the vote among GOP voters in a WMUR poll released Friday. In South Carolina -- the state next in line for the primary -- the former Massachusetts governor is leading the pack with almost 40 percent of the vote, while the only southerner in the contest, Texas Gov. Rick Perry, has fallen to 5 percent.
Romney is leading well ahead of his rivals in all major national polls, but Santorum is also rising quickly, mostly at the expense of Gingrich. In Gallup's latest tracking poll conducted on Jan. 1-6, 29 percent of GOP voters chose Romney, 17 percent picked Gingrich and 16 percent said they would vote for Santorum. This is one of Romney's largest leads since the poll began.
ABC News' Michael Falcone and Arlette Saenz contributed to this report.