The partisan battle underscores a wide consensus among economists and policy experts that no matter who wins the presidency, tax reform will be near the top of the national agenda before the end of the year, when the Bush-era tax cuts, worth $100 billion a year, are set to expire and lawmakers turn again to addressing the government’s budget problems.

“Scaling back the tax expenditures in the tax code will be incredibly difficult,” said Mark Zandi, the chief economist at Moody’s Analytics. “There is some constituency ready to go to war for every deduction and credit in the code. They’re ready to go on jihad. It will be very, very hard.”

Mr. Romney’s tax proposal is built on three central pillars. He has pledged that he will cut all of the marginal tax rates by 20 percent — so the top tax rate would fall to 28 percent from 35 percent, and the bottom tax rate would fall to 8 percent from 10 percent. That by itself would reduce the government’s revenue by hundreds of billions of dollars a year.

Second, Mr. Romney has promised that his plan will be “revenue neutral,” meaning it would pay for those rate reductions by clearing out the underbrush of loopholes and credits in the tax code.

But each tax break has its own rationale and fierce defenders.

Eliminate the home-mortgage interest deduction (annual cost: $99 billion and rising) and risk that housing prices will plummet just as that sector of the economy is starting to recover. End the deduction for charitable giving (annual cost: $53 billion) and attract the wrath of every hospital chief and museum director. Touch the protections for investment income (annual cost: more than $100 billion) and anger everyone from Wall Street executives to retirees.

Given that reality, the campaign has said it would protect some tax breaks — most notably the home-mortgage interest deduction and the investment protections.

But it has refused to detail which tax expenditures it would cut, leaving economists and other tax experts guessing and making a definitive analysis of its effects on families, businesses and the economy close to impossible.