Here again, the long-term consequences were hard to figure, because Mr. Cohn and Mr. Mnuchin offered no estimates of the plan’s costs; guesswork by some analysts put the figure in the same ballpark as the tax plan Mr. Trump offered during the campaign, or about $7 trillion in additional debt over the first 10 years and nearly $21 trillion by 2036.

Whatever the number, the outcome cannot be good. There are legitimate reasons to run deficits, including lifting the economy in tough times, strengthening the military against proven threats and building or rebuilding public infrastructure. Borrowing trillions of dollars to provide a huge windfall for people at the top is not one of those reasons.

Mr. Trump’s plan aims to cut corporate tax rates from 35 percent to 15 percent. To hear the administration tell it, the present rate is choking investment and killing jobs. In fact, big businesses are earning record profits, and many of them pay no federal taxes. The corporate income tax brought in just 10.6 percent of the federal government’s revenue in 2015, down from between a quarter and a third of revenue in the 1950s, according to the Pew Research Center. A better approach, as part of broad-based reform, would be to eliminate loopholes that have encouraged businesses to avoid their fair share of taxes.

Mr. Trump would also apply that 15 percent tax rate to pass-through income that business owners get from limited liability companies, a change that would directly benefit real estate developers like him. This would also create a huge incentive for wealthy Americans to turn their earnings into pass-through income in order to avoid paying higher personal income tax rates. This is no idle threat. Many Kansas residents, including the men’s basketball coach of the University of Kansas, have sheltered income in L.L.C.s since that state exempted income generated through such legal structures from its income tax in 2012.

In addition to lowering the top individual income tax rate to 35 percent, Mr. Trump would do away with the alternative minimum tax, which accounted for a vast majority of the taxes he paid in 2005, according to his leaked tax return from that year, and is one way of making sure that most well-off Americans pay a significant tax on ordinary income. He would also get rid of the estate tax, benefiting mainly wealthy families like his.