Democrats have no shortage of criticism for the massive Republican policy bill winding its way through Congress carrying President Trump’s agenda. It would cost too much, they contend, rip health coverage and food assistance away from too many people and strip vital support from clean energy companies.

When it comes to some of the tax cuts in the bill, however, Democrats have been less resistant. Some of them concede that they would support many of those provisions if they were not rolled into the larger piece of legislation. In recent weeks, they have taken pains to demonstrate that support.

Last month, Senator Jacky Rosen, Democrat of Nevada, successfully moved to have the Senate unanimously approve a version of Mr. Trump’s “no tax on tips” proposal. While the effort was almost entirely symbolic — under the Constitution, the House must originate tax measures — it was still an opportunity for Democrats to go on the record backing a campaign promise of Mr. Trump’s that is broadly popular with the public.

“I am not afraid to embrace a good idea, wherever it comes from,” Ms. Rosen said on the Senate floor at the time.

The undercurrent of Democratic support for elements of the Republican tax agenda reflects the political potency of some of Mr. Trump’s campaign promises, even those that have been derided by tax policy experts. It also suggests that temporary provisions in the Republican bill, like exempting tips and overtime pay from the income tax, could ultimately become long-term features of the tax code.

And it helps to explain why Mr. Trump and Republicans chose to wrap their policy agenda into one huge bill. By pairing the palatable tax cuts — including an extension of tax cuts set to expire at the end of the year — with less savory measures, like Medicaid cuts, Republicans can make the political case that anyone who fails to support the bill is voting for a tax increase.