On the corporate side, said Roberton Williams, a senior fellow at the Tax Policy Center, different companies pay taxes at very different effective rates because the tax code has so many special breaks, loopholes, credits and allowances that some, but not all, can use. In a simpler system, “those who are most disadvantaged will benefit the most, because they have nothing to lose,” he said.

When it comes to individual income taxes, treating everyone the same can have perversely disproportionate effects.

For example, Mr. Williams said it would be hard for Mr. Romney to cut tax brackets across the board without tilting the advantage toward the richest few. Mr. Romney said today that he would limit deductions for the rich but not for the middle classes in an attempt to avoid that outcome. But his advisers were not spelling out the details, saying they would have to be negotiated with Congress in any event.

The administration plan to revamp a corporate code that is widely derided as inefficient and anticompetitive has been in the works at Treasury for two years, and is a priority of Mr. Geithner. Yet he has been preoccupied with crisis management, and is unlikely to see the project through since he plans to leave office after this year.

“The current tax code was written for a different economy in a different era,” Mr. Geithner said, citing such changes as the Internet revolution, cellphones, the rise of China and other emerging markets and a global trend to lower corporate rates. “It needs to be reformed and modernized.”

Republicans and business groups complain that the 35 percent corporate tax rate is among the highest in the world, leaving American companies at a competitive disadvantage. They typically seek a 25 percent rate, with many of them saying that the current tax breaks should be kept in place as well.

Earlier this year, Mr. Obama proposed to end tax breaks for companies that move jobs overseas and to create new breaks for those that bring jobs back.