Romney made the statement, apparently succumbing to pressure from rivals, to reporters as he campaigned in South Carolina in advance of Saturday's primary, The Washington Post reported.
During the regular press briefing at the White House, spokesman Jay Carney said he thought it is unfair for Romney to be paying an effective tax rate of 15 percent when middle-class taxpayers pay a much higher rate, as much as 35 percent.
"What's the effective rate I've been paying? It's probably closer to the 15 percent rate than anything," Romney said in promising to release his returns. "My last 10 years, I've -- my income comes overwhelmingly from investments made in the past rather than ordinary income or rather than earned annual income. I got a little bit of income from my book, but I gave that all away. And then I get speaker's fees from time to time, but not very much."
Romney said he would wait until he files his taxes for 2011 before releasing the returns.
"Rather than sort of have multiple releases of tax returns, why -- we'll wait until the tax returns for the most recent year are completed [in April], then release them," he said.
"I think you know, from what the president has said and others in the administration, that he believes very strongly that -- or he agrees, rather, with Warren Buffett that those who are making millions of dollars -- millionaires and billionaires, say -- should not pay a lower effective tax rate than middle-class Americans," Carney said when asked about Romney's revelation. "As Warren Buffett put it, he should not pay a much lower tax rate than his own administrative assistant, his own secretary.
"The president shares that belief. And I think that this only illuminates what he believes is an issue, which is that 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 percent when folks making $50,000 or $75,000 or $100,000 a year are paying much more. He thinks we ought to fix that."