"I have nothing in them that suggests there's any problem and I'm happy to do so," he said. "I sort of feel like we're showing a lot of exposure at this point," he added.
Romney came under criticism from Newt Gingrich, Rick Perry and Rick Santorum across two hours in the first of a pair of debates in the run-up to this weekend's first-in-the-South primary in South Carolina. The former Massachusetts governor won the first two events of the campaign, the Iowa caucuses and last week's New Hampshire primary, leads in the pre-primary polls in South Carolina and won an endorsement from campaign dropout Jon Huntsman earlier in the day.
Gingrich has virtually conceded that a victory for Romney in South Carolina would assure his nomination as Democratic President Barack Obama's Republican rival in the fall, and none of the other remaining contenders has challenged that conclusion.
That only elevated the stakes for Monday night's debate, feisty from the outset as former House Speaker Gingrich, Texas Gov. Perry and former Pennsylvania Sen. Rick Santorum sought to knock Romney off stride while generally being careful to wrap their criticism in anti-Obama rhetoric.
"We need to satisfy the country that whoever we nominate has a record that can stand up to Barack Obama in a very effective way," said Gingrich.
The five men on stage also sought to outdo one another in calling for lower taxes.
Texas Rep. Ron Paul won that competition handily, saying he thought the top personal tax rate should be zero.
In South Carolina, a state with a heavy military presence, the tone turned muscular at times.
Gingrich drew strong applause when he said: "Andrew Jackson had a pretty clear idea about America's enemies. Kill them."
Perry also won favor from the crowd when he said the Obama administration had overreacted in its criticism of the Marines who were videotaped urinating on the corpses of Taliban fighters in Afghanistan.
The former House speaker and Perry led the assault against Romney's record at Bain Capital, a private equity firm that bought companies and sought to remake them into more competitive enterprises, with uneven results.
"There was a pattern in some companies ... of leaving them with enormous debt and then within a year or two or three having them go broke," Gingrich said. "I think that's something he ought to answer."
Perry referred to a steel mill in Georgetown, S.C. where, he said, "Bain swept in, they picked that company over and a lot of people lost jobs there."
Romney said that the steel industry was battered by unfair competition from China. As for other firms, he said, "Four of the companies that we invested in ... ended up today having some 120,000 jobs.
"Some of the businesses we invested in were not successful and lost jobs," he acknowledged.
It was Perry who challenged Romney, a multimillionaire, to release his income tax returns. The Texas governor said he has already done so, adding he believes Gingrich will do likewise later in the week.
"Mitt, we need for you to release your income tax so the people of this country can see how you made your money. ... We cannot fire our nominee in September. We need to know now."
Later, a debate moderator pressed Romney on releasing his tax returns.
His answer was anything but crisp.
"But you know if that's been the tradition I'm not opposed to doing that. Time will tell. But I anticipate that most likely I'm going to get asked to do that in the April time period and I'll keep that open," he said.
Prodded again, he said, "I think I've heard enough from folks saying look, you know, let's see your tax records. I have nothing in them that suggests there's any problem and I'm happy to do so. I sort of feel like we're showing a lot of exposure at this point, and if I become our nominee and what's happened in history is people have released them in about April of the coming year and that's probably what I'd do."
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.