Many Republicans have claimed that the tax bill won’t cost the government any extra money. The Treasury Department put out a one-page paper on the Senate-passed version that said it would be paid for by economic growth. But the threadbare analysis was able to come to that conclusion only by assuming that Congress passed other legislation, such as an infrastructure bill.

All nonpartisan analyses say otherwise. The conservative Tax Foundation found that the Republican bill will reduce government revenues by $1.47 trillion over a decade, and even assuming growth helps cover the cost it would still come up $448 billion short. The Penn Wharton Budget Model finds that it will cost at least $1.8 trillion over 10 years. The nonpartisan Joint Committee on Taxation found that the Senate-passed version — which differed a bit from the final legislation, but not enough to radically change the numbers — would not pay for itself, costing about $1 trillion.

If these models are right and the bill does add to the deficit, Republicans have already said they’ll cut government spending further.

Jeb Hensarling, Republican of Texas, has insisted that the cost of the tax bill will be covered by economic growth. But, he told Bloomberg, “If not, as a Republican, the answer would be less spending, not more tax.”

Others have admitted that no matter what happens, they’ll call for spending cuts. Marco Rubio told an audience in late November that tax reform is only one side of the Republican plan; the other is to reduce government programs: “You are still going to have a debt problem in the absence of spending cuts.”

Mr. Ryan sounded the same note, saying, “We’ve got a lot of work to do in cutting spending.” Mark Sanford, Republican of South Carolina, was even more blunt. Will the tax bill “help on the margin? Yes. Will it do as much as people advertise? Probably no,” he said. “The real conundrum that we still have to deal with if you really care about debt and deficit is spending.”