At issue with carried interest is the tax code’s longstanding favorable treatment of the generous compensation given to hedge fund managers, private equity partners and venture capitalists as their share of the gains on money they invest for others — institutions as well as wealthy partnerships. That compensation, typically 20 percent of any investment gains (with an additional fee of 2 percent of the sum under management), is taxed as capital gains rather than ordinary income. That means a tax of 23.8 percent — instead of the up to 39.6 percent for ordinary income — even though these money managers generally have not put their own money in play. The traditional justification for low capital gains taxes is to reward risk-taking investors.

Congressional Democrats have tried in the past to make that compensation taxable at the higher “ordinary income” rates paid by wage earners; Mr. Obama has also proposed as much in his budgets. The carried-interest break was an issue in the 2012 election, given its benefit to the Republican nominee Mitt Romney, a founder of the private equity firm Bain Capital. Mr. Romney’s campaign, on the defensive, suggested that as president he would end the break as part of a rewrite of the tax code.

Postelection, the issue faded again. Then in late August, Mr. Trump, on the CBS Sunday morning program “Face the Nation,” said, “The hedge fund guys are getting away with murder,” tax-wise. Defying Republican political gravity, after his heresy of proposing a tax increase, Mr. Trump’s popularity among Republicans rose.

Image Grover Norquist, founder and president of Americans for Tax Reform. Credit... T.J. Kirkpatrick/Getty Images

Two weeks later on the same program, Mr. Trump doubled down. He claimed the hedge fund industry “totally controlled” both Mr. Bush and the Democratic presidential candidate Hillary Rodham Clinton (who opposes the carried-interest break), and added that, under him, hedge fund managers are “going to be paying up.”

With their counterattack gearing up this week, carried-interest defenders were resting easier. On Wednesday, Mr. Ellis said that his intelligence gathering suggested that Mr. Trump’s tax proposal would not be as troublesome as the candidate’s rhetoric suggested. Mr. Ellis also predicted that, given Democrats’ rush to join Mr. Trump’s opposition to the tax break, “we’ve seen the last Republican presidential campaign that’s for ending it.”

But hours later, remarks at the Republican presidential debate on CNN suggested that Mr. Ellis’s information was faulty. Before a vast television audience, Mr. Trump said that when he released a comprehensive tax-overhaul plan in two weeks, “the hedge fund guys won’t like me as much as they like me right now.