In speech after speech on the Senate floor, Mr. Rubio made impassioned arguments for the expansion.

“These are working people, the backbone of our country, the people who have suffered the most over the last 25 or 30 years, as the economy has made some people very profitable but left far too many American workers behind,” he said in one speech earlier this month. “Their anxieties, their daily concerns, the challenges they are facing really underpin a lot of the anxiety in our country, both electoral, political, and economic.”

His amendment went down in flames, with just a handful of Republicans supporting it.

The story could have ended there. But days later, Mr. Trump signaled his willingness to go higher on the corporate rate, and House and Senate negotiators agreed this week to set the rate at 21 percent and lower the tax rate on the wealthiest Americans to 37 percent from 39.6 percent.

Mr. Rubio said that irked him.

“How it bugged me is, when I proposed 20.94, the argument was that this would be catastrophic, and this would be anti-growth,” he said. Twenty percent “was the best thing in the world, but 21 would make it noncompetitive. And yet, 10 days later, 21 was perfectly acceptable.”

He called the expansion of the child credit “a significant step forward,” but said it is only one part of “a broader effort, in my mind, to reform the conservative movement, the Republican Party and our national policies toward being more pro-worker, working family and not just pro-growth.”

Critics of Mr. Rubio — who note that he also threatened to vote against the repeal of the Affordable Care Act, before agreeing to vote for it — say he was posturing, and that the final measure fell far short of what he and Senator Mike Lee, the Utah Republican who also backed expanding the credit, demanded.

“Ten million children in low-income working families will get nothing from the last-minute changes to the G.O.P. tax bill’s child tax credit increase — and as a result will get just a token increase of $75 or less per family,” said Chye-Ching Huang, the deputy director of federal tax policy for the nonpartisan Center on Budget and Policy Priorities.

“Marco Rubio’s ‘great victory’ leaves out 10 million kids,” Michael S. Linden, a fellow at the Roosevelt Institute, a left-leaning policy organization, wrote on Twitter.