The way most economists “score” a tax proposal is to ask how it would change revenue levels compared to what you would expect the government to collect if the tax cut had not passed — what economists call a “baseline.”

In the summer of 2017, for example, the budget office projected that the economy would grow by 2 percent in the 2018 fiscal year, and that personal, corporate and payroll taxes would add up to $3.24 trillion. Then the tax cuts passed, growth accelerated and, for the 2018 fiscal year, tax revenues fell $183 billion — or 5.6 percent — short of that projection.

Republicans, particularly in the Trump administration, sold the tax law on claims that it would pay for itself — even when economists outside the administration, like the congressional Joint Committee on Taxation, released models contradicting them. As corporate tax receipts fell significantly last year, some Republicans began to insist that, in fact, the bill was paying for itself, because total tax revenues were very slightly up.

The 2018 figures contradict that argument, too.

The uncomfortable truth for the bill’s supporters is that the tax cuts are substantially contributing to a widening federal budget deficit, which now appears on track to top $1 trillion this year. If growth fades in the coming years — as many economists believe it will — the cuts could exacerbate the deficit even more .

The best-case scenario for proponents is that the cuts spur a sustained increase in productivity and growth, which in turn produces increasingly higher revenues several years down the road — enough to reduce the “cost” of the bill to the budget deficit.

The 2018 results are, oddly enough, what a lot of economists predicted would happen with Mr. Trump’s cuts, including ones who generally favor tax cuts. Total federal revenues in 2018 came in roughly where the Tax Foundation, a Washington think tank that typically projects large growth boosts from tax cuts, had forecast — which is to say, well below the budget office’s baseline.