Afterward, Gingrich said that wasn't good enough. "If there's nothing there, why is he waiting till April?" the former House speaker told reporters.
Santorum stayed away from the clash over taxes, instead starting a dispute of his own. He said a campaign group supporting Romney has been attacking him for supporting voter rights for convicted felons, and asked Romney what his position was on the issue.
Romney initially ducked a direct answer, preferring to ask Santorum if the ad was accurate.
He then said he doesn't believe convicted violent felons should have the right to vote, even after serving their terms. Santorum instantly said that as governor of Massachusetts, Romney hadn't made any attempt to change a law that permitted convicted felons to vote while still on parole, a law that the former Pennsylvania senator said was more liberal than the one he has been assailed for supporting.
Romney replied that as Republican governor, he was confronted with a legislature that was heavily Democratic and held a different position.
He also reminded Santorum that candidates have no control over the campaign groups that have played a pivotal role in the race to date.
"It is inaccurate," Santorum said of the ad assailing him, seeking the last word. "I would go out and say, 'Stop it. That you're representing me and you're representing my campaign. Stop it.'"
That issue returned more than an hour later, when Gingrich said he, too, faces false attacks from the same group that is criticizing Santorum. He noted that Romney says he lacks sway over the group, "which makes you wonder how much influence he would have if he were president."
Romney said he hoped no group would run inaccurate ads, and he said the organization backing Gingrich was airing a commercial that is so false that "it's probably the biggest hoax since bigfoot."
He called for scuttling the current system of campaign finance laws to permit individuals to donate as much money as they want to the candidates of their choice.
Noting that the debate was occurring on Martin Luther King Jr. Day, one moderator asked Gingrich if his previous statements about poor children lacking a work ethic were "insulting to all Americans, but particularly to black Americans?
"No," he said emphatically, adding his aim was to break dependence on government programs.
"I'm going to continue to find ways to help poor people learn how to get a job, learn to get a better job and learn someday to own the job," he said.
Romney is the leader in the public opinion polls in South Carolina, although his rivals hope the state's 9.9 percent unemployment rate and the presence of large numbers of socially conservative evangelical voters will allow one of them to slip by him.
Huntsman was the second campaign dropout to endorse Romney, after former Minnesota Gov. Tom Pawlenty. Minnesota Rep. Michele Bachmann, who quit after a last-place finish in Iowa, has not yet said which of the remaining contenders she supports. Herman Cain, who left the race in December after facing allegations of sexual impropriety, has promised an endorsement soon.
Huntsman's parting announcement included a reference to the differences he and Romney had. But he left the podium without responding to questions about his remark last week, in the run-up to the New Hampshire primary, that Romney was unelectable and out of touch.
It was unclear why Romney did not attend the announcement. He was in town for a later campaign appearance and then the debate.