Ms. Collins also announced her support, after conversations with business owners in her state and with Mr. Holtz-Eakin, though she came away with the impression that the bill would pay for itself. Other wavering senators signed on — all but Mr. Corker, who was alone in being rattled by a Joint Committee on Taxation analysis that showed the bill would add $1 trillion to deficits even after accounting for additional growth. His lone no was not enough to stop the bill, even when coupled with every Democrat in the chamber, though it gave Republicans a slim margin for error. Mr. Corker later reversed course and said he would support the final version.

Choosing to negotiate with Republicans

As the bill raced through Congress, it sank in the eyes of the public. Majorities of Americans told pollsters they opposed it, and that they expected it would raise, not lower, their tax bills. Republicans told each other those polls would flip — that the country would come to love the bill when it saw its benefits.

Democrats fanned the dissatisfaction, with constant complaints about the bill and its process. Mr. Schumer and Ms. Pelosi thundered on the Senate and House floor that the bill would hurt middle-class Americans, clearly setting up a campaign theme for Democrats to embrace in the midterm elections.

Senator Ron Wyden of Oregon, the ranking Democrat on the Finance Committee, said he started the year with a sense of cautious optimism about tax policy but found Republicans unwilling to engage in a serious way.

Mr. Wyden described a visit from Gary D. Cohn, the director of the National Economic Council, as a “show and tell” and said that Mr. Mnuchin, the Treasury secretary, never followed up on a promise to look into ways to make the tax plan more populist.

Mr. McConnell and Mr. Toomey lamented that the bill was not bipartisan and thus was less likely to be enduring. Mr. Toomey said that Republican leaders talked seriously about working across the aisle, but that when Democratic senators sent a letter last summer with strict conditions for working with them, it was clear that Republicans would have to proceed on their own.

The final negotiations this month were entirely between Republicans. The Senate version of the bill largely won out, but House leaders pushed, successfully, for an immediate cut of the corporate rate, which was raised slightly to 21 percent from 20 percent, and for a reduction in the top individual tax rate to 37 percent.