Ms. Brescher’s confusion is hardly surprising. Mr. Trump and congressional Republicans promised a broad-based tax cut, particularly for the middle class. The emerging bill, however, is a web of complex provisions that would cut taxes for some people, raise them for others and have different effects in different years. Even families that look similar on paper could be affected differently depending on where they live and how they earn their money.

“It comes down to a bunch of idiosyncratic factors,” said Mark Mazur, a Treasury Department official in the Obama administration who now directs the Tax Policy Center. “You really have to parse through the details.”

Sentiment about the bill seems strongly colored by feelings about the president. Backing remains strong among supporters of Mr. Trump: 80 percent of them say they approve of the bill. Among those who don’t approve of Mr. Trump, the figure is only 10 percent.

Geoffrey Cantley, an Army recruiter in rural Virginia, said he didn’t know whether he’d get a tax cut or a tax increase under the bill. But he said Mr. Trump’s efforts to simplify the tax code would enable him to file without a tax preparer, saving money. And he said Mr. Trump deserved the benefit of the doubt.

“I don’t think he’s getting a fair shake in the coverage he’s getting,” Mr. Cantley said. “I think the economy’s doing a lot better.”

Republicans said this week that public doubts would fall away once the bill becomes law and tax cuts begin appearing in paychecks. They cited polls from 1986, showing that most Americans did not feel that year’s tax bill would help them, either.

“Whatever the polling data is that’s out there today doesn’t recognize just how powerful this bill is going to be to put more money in the pockets of hard-working families,” said Representative Steve Scalise of Louisiana, the majority whip.