Corporate Tax

Ginsberg added that "much of the income" subject to the preferential rate on investments has already been taxed at rates of up to 35 percent in the corporations in which they were earned.

Romney's return indicates that he carried forward $4.8 million in capital losses from previous years, an indication that he didn't report positive capital gains on his 2009 return. The campaign didn't release returns from before 2010.

"It's an extensive disclosure and we feel it satisfies" the requests for Romney to release his returns, Ginsberg told reporters today.

Seth McKee, a political science professor at the University of South Florida in Tampa, said the campaign's handling of the tax-return issue was a "terrible blunder" that may draw unwanted attention to the candidate's wealth.

Buffett Blames Congress

"You created the problem yourself," he said. "Once it's out there, it won't be as big an issue. We'll move onto something else."

Billionaire investor Warren Buffett, who is calling for raising taxes on high-income Americans, said he blames Congress, not Romney, for the governor's tax rate.

"It's the wrong policy to have," Buffett told Bloomberg Television's Betty Liu in an interview yesterday. "He's not going to pay more than the law requires, and I don't fault him for that in the least. But I do fault a law that allows him and me earning enormous sums to pay overall federal taxes at a rate that's about half what the average person in my office pays."

In 2008, according to IRS statistics, fewer than 15 percent of taxpayers earning more than $200,000 had effective tax rates of less than 15 percent.

Romney, who lost the South Carolina primary Jan. 21, is competing in the Jan. 31 contest in Florida against his main rival, former House Speaker Newt Gingrich.