The highest earners -- including Romney -- would benefit most from several of his proposals that differ from the Bowles- Simpson approach. Those include repealing the estate tax, cutting the corporate tax rate to 25 percent from 35 percent, and keeping the 15 percent tax rate on capital gains and dividends.

The Tax Policy Center estimated how Romney's policies would affect different income groups, without the reductions in tax breaks he has yet to spell out.

Under Romney's plan as announced, the top 20 percent of taxpayers would receive an average tax cut of $16,134, compared with what they are paying now. The top 0.1 percent -- those with incomes exceeding $2.9 million in 2015 -- would get an average tax cut of $725,716.

Middle-Class Burden

An estimate being released this week by the Brookings Institution's Hamilton Project shows how hard it is to cut the top income tax rate without losing revenue or causing the middle class to pay a larger share.

To reduce the top rate to 23 percent -- below Romney's 28 percent target -- and generate today's revenue would require eliminating almost every tax break, including those for employer-sponsored health insurance, retirement savings and municipal bonds, capital gains and dividends.

A 27 percent top rate would allow some benefits for mortgage interest and health insurance to remain.

"You can only get rates down so far before you start losing revenues or really changing the progressivity of the tax schedule," said Adam Looney, the Hamilton Project policy director, who helped develop the analysis. He was an economist at the Federal Reserve Board and the White House Council of Economic Advisers under Obama.

Larger Share

The current income tax system requires higher-income people to pay a larger share of taxes. In 2009, the top 10 percent of taxpayers received 43.2 percent of adjusted gross income and paid 70.5 percent of income taxes. Their percentage of taxes paid is lower if the calculation includes payroll taxes, which pay for Social Security and Medicare.

Ending the tax preference for investment income would reduce the after-tax income of the top 0.1 percent by 7.5 percent, compared with 0.9 percent for all Americans, according to the Tax Policy Center.

A Romney-style plan "theoretically could be progressive between the upper-middle class and the affluent, compared to the middle class," said Edward Kleinbard, former chief of staff at the congressional Joint Committee on Taxation. "But it will not address the progressivity of the super-affluent with a 15 percent tax rate."