Florence, South Carolina (CNN) – Mitt Romney said Tuesday he pays roughly a 15% effective tax rate on his income - an acknowledgment that the multi-millionaire pays a smaller percentage of taxes on his income than many middle-income Americans.
"It's probably closer to the 15 percent rate than anything," Romney told reporters when asked about his effective tax rate. "The last ten years my income comes overwhelmingly from investments made in the past rather than ordinary income or rather than earned annual income."
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The information came as his opponents ratcheted up pressure on the GOP front-runner to release his tax returns in a debate Monday night – a disclosure that is not required by law and one Romney has been reluctant to make. Some Americans of far more modest means pay a 15% marginal tax rate on income, while the highest tax bracket tops out at a 35% marginal rate. Because Romney's income comes primarily from investments, it is taxed at a much lower rate.
The candidate, a former chief of private equity firm Bain Capital, went a step further Tuesday and committed to releasing his tax returns in April if he were to become the GOP nominee.
"We'll wait until the tax returns for the most recent year are completed, then release them," he said.
Romney said he had donated the "little bit of income" he earned from his book, "No Apologies," and said occasionally he had been paid fees to speak at events.
The candidate was also elaborated on his views on super PACs, independent political action committees that can raise unlimited funds to advertise on behalf of a candidate. A campaign cannot share strategy with a super PAC.
Romney said he had both encouraged and raised money for his super PAC, and said both former Sen. Rick Santorum and former House Speaker Newt Gingrich had done so as well. He added that although he did not like the existence of super PACs, he would not "disadvantage" his campaign by not taking advantage of the process.
"I wish that law were not as it is. I wish we could just raise money for our campaigns instead of having to create super PACs," Romney said. "I think the whole idea of these PACs becoming larger than the campaigns themselves is a very bad idea."
Gingrich has faulted Romney for failing to publicly denounce attack ads aired by a super PAC supportive of Romney's candidacy, Restore Our Future.
In the press conference, Romney did defend some of the charges made against other candidates by Restore Our Future, including a dig at Gingrich for ethics violations and for his participation with former House Speaker Nancy Pelosi on a climate change awareness initiative, and an ad that charged Santorum supported allowing felons to vote.
"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!" Romney said. "What's he missing? That's his position."
The press conference was not without several light moments. After walking to the microphone, Romney first called on his wife, Ann, who had been talking with reporters.
"Where'd the skinny jeans go?" she asked her husband, who did not answer the question.
Romney also weighed in on a campaign commercial aired by Comedy Central comedian and host Stephen Colbert. The ad jokingly calls Romney a "serial killer."
The candidate said he had not seen the ad but admitted of Colbert: "he has a tradition of being able to touch my funny bone."
CNNMoney's Charles Riley contributed to this report.