Republican leaders and Mr. Corker, who owns a real estate partnership in Tennessee, say the new loophole was not put in place to win over his vote. Mr. Corker has become more important because his party can afford to lose only two votes, and Senator John McCain will be absent because of the aftereffects from his cancer treatment.

Republicans insist, further, that the provision was not “airdropped” — Mr. Corker’s term — into the tax bill during conference committee negotiations, and that its main purpose was to make sure pass-through businesses were not treated unfairly because corporations would be getting a big tax cut to 21 percent, from 35 percent now. Whatever the Republicans’ protestations, this malodorous loophole is further confirmation that congressional leaders are doing everything they can to maximize benefits for the wealthy at the expense of almost everybody else.

As for Mr. Trump, he has been going around saying the tax bill would “cost me a fortune” and his accountants “are going crazy now.” This claim has always been “fake news.” But with the new loophole it has become even more nonsensical. Having done nothing to drain the Washington swamp, the president now luxuriates in its warm waters.

All told, the 20 percent deduction for pass-through income would cost the government $414.5 billion in lost revenue over 10 years, according to Congress’s Joint Committee on Taxation. To put that number into context, it is about 29 times as much as the roughly $14 billion a year that the federal government spends on the Children’s Health Insurance Program, which covers nearly nine million kids from low-income families. Congress let authorization for that program lapse at the end of September.

The tax bill’s generosity toward real estate titans stands in stark contrast to its stinginess toward the average wage earner as well as its very real damage to taxpayers in high-cost states. Average wage earners who would get modest tax cuts in the early years would see them evaporate into thin air after 2025. Homeowners and others in high-cost states like California, New Jersey and New York would see their once-sizable deductions for state and local taxes shrink to a maximum of $10,000 a year, which could in turn reduce home values. Further, the tax bill would permanently change how tax brackets are adjusted for inflation so that more people would be pushed into higher tax brackets over time even if they received only modest raises in salary.