When it comes to tax policy, former President Donald J. Trump has appeared in recent weeks to channel Oprah Winfrey, the television legend whose audience giveaways became a cultural sensation.

But instead of free cars, Mr. Trump, the Republican presidential candidate, is offering his audiences bespoke tax cuts. For restaurant and hotel employees in Nevada, a swing state, Mr. Trump offered “no taxes on tips.” For the retired Americans who vote in great numbers, he said he would end taxes on Social Security benefits. For business executives at the Economic Club of New York, Mr. Trump offered a cut in the corporate tax rate to 15 percent, from 21 percent, for domestic manufacturers.

And for his fellow New Yorkers, whom Mr. Trump addressed at a rally in Long Island on Wednesday, he said he would bring back a prized deduction for state and local taxes, known as SALT, a tax break that he once limited.

“Going to restore SALT,” he said during his rally, adding that the move would “save thousands of dollars for residents of New York, Pennsylvania, New Jersey.”

After reading out a list of House Republicans in attendance at his rally, Mr. Trump pointed to that group, including Representative Anthony D’Esposito of New York, saying: “SALT, Anthony. Remember, fellows, SALT.” Mr. D’Esposito stood and gave Mr. Trump a thumbs-up.

Earlier in the night, he promised that “if you vote for me, I’m going to reduce your taxes.”

The plans, so far offered as pithy one-liners without detailed explanations, have proven politically popular, energizing crowds at his rallies and in one instance even prompting his Democratic rival to adopt a version of his idea. They have also scandalized tax and budget experts in Washington, who balk at the mounting cost of Mr. Trump’s anti-tax crusade.

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