Retaining the current progressive tax burden without changing capital gains rates is difficult, if not impossible, said Eric Toder, co-director of the Tax Policy Center.

"What you'd have to do is tax gains and dividends as ordinary income, because that's where the very big preferences are," he said. "That's just the math."

'Aggressive' On Deductions

For the group between the top 1 percent and the bottom 50 percent, the outlines of Romney's plan are more achievable.

"There's a huge amount of money that can be thrown back into the pot if you're willing to be very aggressive about personal itemized deductions," said Kleinbard, now a law professor at the University of Southern California in Los Angeles.

Repealing all itemized deductions would reduce after-tax income by 1.7 percent, by 2.7 percent for the top 20 percent, and by 3 percent for the top 1 percent of taxpayers, according to the Tax Policy Center. That means those breaks are less tilted toward the highest-income taxpayers than the preference for investment income.

Romney made his clearest comments about broadening the tax base at an April 15 fundraiser, when reporters overheard him talking about eliminating the state and local tax deduction and the mortgage interest deduction for top earners' second homes.

'Up Their Sleeve'

Those changes wouldn't be nearly enough to make up the revenue loss from his proposed tax cuts.

"I just don't see where the numbers add up," Toder said. "Maybe they've got something up their sleeve I haven't seen."