“The evil party and the stupid party got together and called it bipartisan,” Brian Riedl, a senior fellow in budget, tax and economics at the conservative Manhattan Institute for Policy Research, said of the big budget deal. “This is the beginning of a long-term avalanche caused by Social Security and Medicare costs that are only going to get worse every year. I project $2 trillion within a decade or $3 trillion if interest rates return to 1990s levels. So no, the tax cuts will not pay for themselves.”

But tax cuts have been the single major policy success under the Trump administration in Congress, and both continue to say they will be a net positive for the United States Treasury. They are so confident, in fact, that many Republican lawmakers would like to pass another tax cut that would make many of the reductions in the first round permanent. Mr. Trump’s sweet spot has never involved big changes to entitlement programs, even though the biggest fiscal hawks in Congress remain hopeful that he will get there.

“I think this issue still resonates with Republican voters and independent voters,” said Senator Patrick J. Toomey, Republican of Pennsylvania, who led an unsuccessful charge to make big changes to Medicaid during Republican attempts last year to repeal the Affordable Care Act. “Individual candidates may well be focused on other issues, but there are many of us who still feel very strongly about this.”

As to whether Mr. Trump will be helpful going forward on that front, “it’s unclear,” Mr. Toomey said. “I think the president did indicate he was not interested in taking on big entitlement programs early on. Under the right circumstances, I think he could be an ally.”

The net result for Mr. Ryan, now the retiring speaker of the House, is that he will leave a legacy absent the signature issues that propelled his entire legislative career. “This Congress ended up being about tax reform,” Mr. Cooper said. “It was the low-hanging fruit because there was a Republican coalition of Trump Republicans and establishment Republicans that united around tax reform.”

The scores of Republicans who ran on the issue in 2010 and 2012 have largely become quiet about debt so as to not run against the Trump agenda, even as the tax cut has not been popular enough to use as a re-election case. So incumbents have turned to things like national security instead.

“We have always believed candidates need to represent their states and districts,” said Sal Russo, co-founder of Tea Party Express. “So we are not troubled when someone votes crosswise with us if they believe that it’s necessary to adequately represent their constituents. Thus, we will support candidates around a broad range of issues.”