When Republicans last set out to change taxation in America, they spent years combing through the details of the internal revenue code. They traveled the country, held hearings and drafted early versions of a bill, eventually passed in 2017, that they hoped would transform a sclerotic tax system with long-held conservative principles.

This time around, as Republicans prepared for another opportunity to change how taxes in the world’s largest economy are collected, their core ideas came not from a Washington think tank or corporate accountant. Instead, in President Trump’s telling, a waitress at his hotel in Las Vegas complained to him about having to pay taxes on her tips while he dined there during the 2024 campaign.

Soon, the seemingly offhand remark became a centerpiece of Mr. Trump’s successful campaign back into office. Republicans on Capitol Hill embraced the idea, too, and Congress this week voted to create a new tax exemption for tipped income for the next few years. At an event at the White House last month promoting the legislation, Mr. Trump credited the waitress with helping him win Nevada, where many people work for tips.

“A legend was made,” Mr. Trump said. “We won Nevada by so much. Republicans don’t win Nevada. We won Nevada. So I want to thank that young, beautiful waitress. Thank you very much.”

The tips provision, while ultimately only a sliver of the sprawling package that lawmakers passed this week, marked an important evolution in how the Republican Party, long dedicated to lowering taxes, has approached that goal. Rather than the type of systematic re-examination of the tax code that took place in 2017, the new Republican bill introduces a series of novel, populist and temporary cuts that Mr. Trump cooked up during the 2024 campaign to try to win the support of key constituencies.

At the same time, even as Mr. Trump’s return to office pointed the party’s tax agenda in a more populist direction, the new bill is in many ways the apotheosis of a traditionally conservative, supply-side philosophy. Once Mr. Trump signs the legislation, which he is expected to do on Friday, many of the tax cuts made in 2017 will be the law of the land for the foreseeable future, rather than just temporary features.