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(CNN) -- Republican presidential front-runner Mitt Romney revealed Tuesday that he pays about a 15% income tax rate but continued to resist calls by his rivals to release his tax returns.
Romney said his annual income is "overwhelmingly from investments" rather than ordinary income, which is taxed at a higher rate. Former House Speaker Newt Gingrich joked that he should rename his plan for a single 15% tax rate the "Mitt Romney flat tax," and Texas Gov. Rick Perry said if Romney reveals his tax records it would let voters see "if he's as good a businessman as he says he is."
The White House chimed in as well, with spokesman Jay Carney calling the disclosure of tax forms "an established tradition for presidential candidates" started by Romney's father.
Romney, the millionaire former Massachusetts governor, has faced growing calls to release details of his finances ahead of Saturday's GOP primary in South Carolina. Gingrich and Perry in particular have targeted his background as a former head of Bain Capital, accusing the investment firm of making vast profits while gutting payrolls.
"He's running as an astute businessman," Perry, who trails the pack in recent polls, told CNN's "The Situation Room." His financial records "would be a good thing for the people of South Carolina to take a look at and see if he's as good a businessman as he says he is."
Meanwhile, buoyed by the cheers he received at a Monday night GOP debate, Gingrich told supporters that if he can overtake Romney in the state, "I will be the Republican nominee."
"If I'm the Republican nominee, we will run a campaign of paychecks versus food stamps, and we will beat Obama virtually everywhere in this country," he said at a state Chamber of Commerce forum in Columbia. "There will be no safe state, and it will be a very different election than anybody expects."
Gingrich won applause at the Monday night event for defending his description of Obama as the "food stamp president" against accusations that it has racist undertones, saying the criticism comes from the "politically correct."
South Carolina has picked the winner of every GOP presidential nomination fight since 1980. Gingrich, who finished a distant fifth in more moderate New Hampshire, argued Tuesday that he is the lone conservative capable of topping Romney in South Carolina, where a poll published last week showed him in a statistical tie with Romney.
Romney won the party's first two presidential contests, in Iowa and New Hampshire, and is hoping to score a knockout blow in South Carolina. Tuesday, he said he would probably release his taxes when his 2011 return is ready in April.
"The last 10 years, my income comes overwhelmingly from investments made in the past, rather than ordinary income or rather than earned annual income," Romney said. "I got a little bit of income from my book but I gave all of that away. And then I get speaker's fees from time to time, but not very much."
At the White House, Carney told reporters that President Barack Obama believes wealthy Americans "should not pay a lower effective tax rate than middle-class Americans."
"Everybody who's working hard ought to pay their fair share, and that includes millionaires who might be paying an effective tax rate of 15% when folks making $50,000 or $75,000 or $100,000 a year are paying much more," Carney said, adding that Obama "thinks we ought to fix that."
When a reporter noted Romney was following the law, Carney responded: "The president believes we ought to change the law, for that reason." And he said the tradition of presidential candidates from both parties revealing their tax returns dates back to Romney's father, George, who released 12 years of tax records when he sought the Republican nod in 1968.
Gingrich argued that the party needs Romney to come clean with anything negative before the GOP picks its standard-bearer, "because I guarantee you, Barack Obama's billion-dollar campaign is going to put pressure on anyone who is the nominee."
Meanwhile, Romney fought back against attacks by his rivals during a debate the night before, defending his revised figure for jobs created by his former venture capital firm and denied that an ad by a super PAC supporting him is inaccurate.
At a brief news conference in Florence, Romney repeated his assertion from Monday night's debate that four companies that Bain Capital invested in under Romney's leadership created 120,000 jobs.
Other companies involved with Bain ended up losing about 10,000 jobs, Romney said, insisting the overall record reflected positive job growth as part of the free-market system.
"Do the math," said Romney, who wore a sport coat over blue jeans for the news conference and an earlier appearance at a lightly attended campaign event.
Previously, Romney had said 100,000 jobs were created during his days at Bain Capital. The revised figure reflected an effort to counter attacks by some rival candidates, and ads paid for by super PACs supporting them, claiming that the company raided businesses for profits before shutting them down to put people out of work.
Meanwhile, one of Romney's conservative challengers, former Pennsylvania Sen. Rick Santorum, accused the former Massachusetts governor of dirty politics for refusing to repudiate an ad by a super PAC that accuses Santorum of supporting voting rights for people convicted of felonies.
Seemingly at a loss for words, Santorum told a town hall on the USS Yorktown: "I am stunned that -- that -- that Mitt Romney doesn't have the ability to discern something that is blatantly false and that blatantly gives a false impression."
In particular, the ad shows a uniformed prisoner, while Santorum said the proposal he voted for in the past only applied to convicted felons who served their time and were released back into society.
At his news conference, Romney brushed off the complaint, saying anyone convicted of a violent felony is labeled a felon.
"I hear that Rick Santorum is very animated that the super PAC ad says that he is (in) favor of felons voting. Well, he is! What's he missing?" Romney said.
Santorum and Gingrich are vying to be the surviving conservative candidate to combat the more moderate Romney in a one-on-one battle for the Republican nomination. Complicating the equation is the campaign of Texas Rep. Ron Paul, who finished a strong second in New Hampshire, and the continued presence of Perry, a hardline conservative, even though he lags well behind.
Both Santorum and Gingrich have said they wish the others would drop out to give a conservative champion the best chance to derail Romney's campaign. Santorum criticized Romney as a manager who lacked the inspiring touch of GOP icon Ronald Reagan.
"A lot of those folks voted for Reagan's proposals not because they wanted to, it's because the American public was so far behind what he wanted to do that they had no choice but to," Santorum said. "I don't see anything in Gov. Romney's tool box or experience that would lead me to believe that he's able to do that."
For those voters who don't meet the candidates face-to-face, the South Carolina airwaves were filled with ads from their campaigns and the groups supporting them as they seek to land a fatal blow.
So far, super PACs supporting various candidates or causes have spent at least $6 million on the South Carolina primary and more than $26 million overall, according to federal campaign records.
The groups, which were created following a pair of federal court decisions in 2010, are permitted to receive unlimited campaign contributions from corporations, labor unions, and wealthy individual donors.
Traditional federal political action committees and candidate campaign committees may only receive limited contributions from individuals, political parties, and other political action committees. Under federal law, candidates and super PACs are not permitted to coordinate campaign activity.
Topping the list of big-spending super PACs is Restore Our Future, run by supporters of Romney, which has invested at least $7.8 million in the Republican primary contest. Almost all was for television ads and direct mail in Iowa, South Carolina and Florida attacking Gingrich.
A group supporting Gingrich, Winning our Future, is the second-highest spending presidential super PAC at $4.3 million since just before Christmas. Perry has been the beneficiary of almost $4 million in spending by Make Us Great Again, while Paul has the support of at least two super PACs that have spent a combined $3.3 million.
Romney's campaign is bolstered by a huge war chest and has gained momentum after his early victories.
Gingrich has tried to make the case that a vote for Santorum was essentially a vote for Romney because it would split conservatives and hand the primary to the most moderate candidate still in the race. Santorum, however, pointed out that he had beaten the former speaker in the first two contests in Iowa and New Hampshire.
Seeking to regain the luster of a razor-thin second-place finish behind Romney in Iowa, Santorum has won the endorsement of Christian conservative groups to bolster his support from South Carolina's powerful evangelical voting bloc.
Meanwhile, Texas Rep. Ron Paul's campaign took in $1.3 million in a two-day, online fundraising drive over the weekend that followed the libertarian champion's strong second-place finish in New Hampshire.
Paul is running third in South Carolina, behind Romney and Gingrich, according to poll results. While his support has been consistent, he is considered unlikely to attract the broader backing necessary to eclipse Romney.
Reporters repeatedly ask Paul if he might wage an independent campaign as a third-party candidate in the event he fails to get the Republican nod.
"I don't give anybody the satisfaction of saying I've conceded the race and I am going to go off and do something," Paul said Tuesday.
CNN's Tom Cohen, Robert Yoon, Rachel Streitfeld, Jim Acosta and Adam Aigner-Treworgy contributed to this report.