“If you write a book, and it takes three or four years before you earn a royalty, that doesn’t make the income a capital gain,” he continued. “If a movie takes three years to generate a return, that doesn’t make it a capital gain. There’s no reason why financial services should be any different.”

Proponents of the loophole, most of whom benefit from it one way or another, have long argued that the lower tax rate encourages risk taking and hence economic growth, an argument that Republican lawmakers appear to have embraced.

But there’s little empirical support for the position.

Mr. Fleischer argues that the proposed tax bill — by slashing the corporate and pass-through tax rates and preserving the lower rate for capital gains — already heavily favors risk taking and investment.

“We don’t need to further subsidize people who go to work in this sector by giving them a lower tax rate,” he said. “There’s no shortage of people who want to go into private equity and earn millions or billions of dollars.”

The rationale, he said, “is absurd.”

That the carried interest provision survived, despite Mr. Trump’s campaign statements and populist hostility to it, appears to be a testament to the enduring influence that Wall Street and wealthy investors exert over both the White House and Congress.

Some of Mr. Trump’s most prominent backers and donors come from the world of private equity, hedge funds and real estate partnerships, among them Stephen A. Schwarzman, chief executive of the powerhouse firm Blackstone; Wilbur Ross, the former private equity executive who is now secretary of commerce; Thomas J. Barrack, Jr., executive chairman of Colony Northstar, a real estate investment trust, and chairman of the president’s inauguration committee; Betsy DeVos, a co-founder of the Windquest Group and now secretary of education; and Stephen A. Feinberg, a co-founder of Cerberus Capital Management and a Trump donor and adviser. And Treasury Secretary Steven Mnuchin and Mr. Trump’s top economic adviser, Gary Cohn, are prominent alumni of Goldman Sachs, which has a range of hedge fund and private equity clients.

More broadly, preserving the break for carried interest fits into the overarching theme of the Republican tax plan: that what is good for investors is good for economic growth, which in turn is good for everyone, including the working poor. That’s why the proposed code is tilted so heavily toward investors rather than people who earn salaries or compensation for services rendered.