WASHINGTON — Mitt Romney's plan to overhaul the tax code would produce cuts for the richest 5 percent of Americans — and bigger bills for everybody else, according to an independent analysis.

The study, released this week, was conducted by researchers at the nonpartisan Tax Policy Center, a joint project of the nonpartisan Brookings Institution and the Urban Institute.

To cover the cost of his plan — which would reduce tax rates by 20 percent, repeal the estate tax and eliminate taxes on investment income for middle-class taxpayers — the researchers assume that Romney would go after breaks for the richest taxpayers first.

They also looked at what would happen if Republicans' dreams for a tax overhaul came true and the proposal generated significant revenue through economic growth.

None of it helped Romney. His rate-cutting plan for individuals would reduce tax collections by about $360 billion in 2015, the study says. To avoid increasing deficits — as Romney has pledged — the plan would have to generate an equivalent amount of revenue by slashing tax breaks for mortgage interest, employer-provided health care, education, medical expenses, state and local taxes, and child care, all breaks that benefit the middle class.

"It is not mathematically possible to design a revenue-neutral plan that preserves current incentives for savings and investment and that does not result in a net tax cut for high-income taxpayers and a net tax increase for lower- and/or middle-income taxpayers," the study concludes.

Even if tax breaks "are eliminated in a way designed to make the resulting tax system as progressive as possible, there would still be a shift in the tax burden of roughly $86 billion a year from those making over $200,000 to those making less" than that.

What would that mean for the average tax bill? Millionaires would get an $87,000 tax cut, the study says. But for 95 percent of the population, taxes would go up by about 1.2 percent, an average of $500 a year.

The Romney campaign declined to address the specifics of the analysis, dismissing it as a "liberal study."

Campaign officials noted that one of the three authors, Adam Looney, of Brookings, served as a senior economist on the Obama Council of Economic Advisers. The other two authors are Samuel Brown and William Gale, both of whom are affiliated with Brookings and the Tax Policy Center.