The bill would raise U.S. tax revenue by $47 billion over the next decade, according to the Joint Committee on Taxation, the nonpartisan scorekeeper for Congress. The estimate assumes that George W. Bush-era income tax cuts would expire as scheduled at the end of the year. If those tax breaks stay in place, the joint committee estimates that the bill would raise $162 billion, according to Whitehouse's office.
High earners including Buffett and Republican presidential candidate Mitt Romney would be "clobbered" by the Buffett rule, Williams said. Measured as a share of adjusted gross income, Romney, the co-founder of Bain Capital LLC, paid a 13.9 percent rate for tax year 2010, according to tax returns released by his campaign.
Corporate chief executives and professional athletes with multimillion-dollar salaries would see less of a change in their tax rate because their pay is taxed as ordinary income. High- income households with a mix of ordinary and capital income have rates between 15 and 30 percent. Their taxes would go up under the Buffett rule although they already pay more than middle- class households do.
Obama has framed his push for the Buffett rule as a fairness issue, and made the case by comparing effective tax rates paid by different income groups.
"If you make more than a million dollars annually, then you should pay at least the same percentage of your income in taxes as middle-class families do," the president said in an April 3 speech in Washington.
A 30 percent tax rate "shouldn't be too much to ask," said Seth Larson, a spokesman for Whitehouse.
"Those who can afford it should pay a little more in taxes," Larson said. "This has been a guiding principle of our tax code for generations, and the Paying a Fair Share Act would simply restore that balance."
There are different ways to calculate effective tax rates, and the way it's done can shape the results, said Jon Bakija, an economics professor at Williams College in Massachusetts.
"It's legitimate to say that there are some millionaires who are paying lower average rates than their secretaries," Bakija said. "It's not a large fraction of millionaires, but it's some."
According to the administration's data, the median effective tax rate for the middle 20 percent of U.S. taxpayers is 13.3 percent, including income, payroll and corporate taxes. For the top 1 percent of taxpayers, the rate is 29.6 percent, according to the 2012 Economic Report of the President. Still, 10 percent of the high-income households have tax rates of 8.7 percent or less.