(Bloomberg News) Republican presidential candidate Mitt Romney's tax plan rests on a set of principles that, taken together, are difficult to reconcile.

Romney wants to reduce individual income tax rates by 20 percent, keep preferential rates for capital gains and dividends, broaden the tax base to limit revenue loss, and retain the tax-burden distribution across income groups.

Those goals are in conflict and will require that Romney consider limiting or eliminating the tax breaks for charitable deductions and home mortgage interest, said Martin Sullivan, contributing editor at Tax Analysts in Falls Church, Virginia.

"As soon as he gets in, he's going to have to start backpedaling big-time on all of his promises," Sullivan said. "It's just not doable under any conceivable, realistic scenario."

Romney hasn't provided many details about his tax plan, such as which tax breaks he would curtail or end. He has suggested that tax benefits might be available to middle-income families though not upper-income families, without defining those income levels.

"We're not going to shift the burden from middle-income people to higher-income people," Romney said in an April 16 interview with ABC News. At the same time, he said, "I'm not looking for tax cuts for the rich."

28 Percent Rate

Romney proposes reducing all individual income tax rates, dropping the top rate to 28 percent from 35 percent and the bottom rate to 8 percent from 10 percent. He would end taxation of investment income for households making less than $200,000 a year and keep the 15 percent rate for capital gains and dividends for taxpayers above those levels. He also would eliminate the estate tax and the alternative minimum tax.

Keeping the tax code's progressive distribution among income brackets may be difficult without compromising some of these objectives. The lower rate on investment income rate is the most significant tax break for the very wealthiest taxpayers, and promising to protect that means that Romney has limited ways to offset the rate cuts for that group.

"The top 5 percent, the top 10 percent, the top 25 percent, we'll look across the code at the various categories of taxpayers and see if they're continuing to pay the approximately the same share that they have in the past," Romney said on CNBC March 7.