Some of these tax subsidies are considered untouchable, like the deduction for employee health care costs. But others are expendable. One of the biggest is the interest deduction. Its elimination would raise $700 billion over 10 years, the Tax Policy Center estimates. Many economists like the idea of eliminating the deduction because it favors borrowing over other forms of raising capital, like issuing stock. But big borrowers like real estate developers love the deduction, and Mr. Trump hasn’t yet said that he’s prepared to scrap it.

Another possibility would be to shift at least some of the tax burden from corporations to shareholders, by raising tax rates on capital gains and dividend income. Many economists favor that approach, because it reduces the double-taxation problem embedded in the current system. (Corporations pay taxes on profits, and then, when these are distributed, shareholders pay.) The added tax burden would fall primarily on affluent taxpayers since they tend to own stocks, helping to rebut the argument that slashing the corporate rate would be a tax cut for the wealthy.

While it’s difficult to estimate how much additional tax revenue would be generated if dividends and capital gains were taxed at the same rate as ordinary income, it would surely be substantial, another big step toward a 15 percent corporate rate.

Taxing ordinary income, capital gains and dividends at the same rate would also greatly simplify the tax code. The notion does, however, run up against Republican orthodoxy that low capital-gains rates stimulate growth.

There are many other ways to raise revenue, of course. House Republicans proposed a border adjustment tax, popular with many pro-growth economists, which powerful retail interests shot down. Most European countries fund their low corporate taxes with some form of a value-added tax, on consumption rather than income. House Republicans have nodded in that direction, but a value-added tax runs afoul of many of the same interests that opposed the border tax, and has never been popular in the consumption-friendly United States.

Environmentalists and many economists would love to see a carbon tax, which would presumably be repellent to an administration that is skeptical of climate change and wants to promote the coal industry.

The point, however, is that if Mr. Trump and Republicans in Congress are serious about a 15 percent rate without vastly increasing the deficit, there are many ways to get there.