Trump’s team says the tax bill will pay for itself. It won’t.

The Senate tax bill would grow the economy by about 0.8 percent over 10 years, offsetting some of the bill’s cost but leaving the overall price tag at about $1 trillion, according to a new report by Congress’s Joint Committee on Taxation, the professional staff tasked with estimating the effects of tax changes.

The report flies in the face of confident assertions by Republicans in the Trump administration and in the Senate that their tax cut plan would pay for itself. “Not only will this tax plan pay for itself, but it will pay down debt,” Treasury Secretary Steve Mnuchin promised. “I think this tax bill is going to reduce the size of our deficits going forward,” Sen. Pat Toomey (R-PA) told reporters in early November.

JCT finds, by contrast, that growth would pay for about a third of the bill’s cost. On a “static” basis before considering growth, the bill costs $1.414 trillion over 10 years. The economic effects of the bill, JCT finds, would increase revenues by about $458 billion.

But this, in turn, is partially offset by the fact that the bill’s cost will lead to increased interest payments on the federal debt. Put it all together and you get a net cost of $1.0067 trillion. In no single year does growth pay for the cut.

What’s worse, JCT’s numbers suggest that Republicans’ decision to let major cuts for individuals expire at the end of 2025 dampens the growth effect of the law. Growth raises $56.1 billion in 2025, but only $38.8 billion the next year. Over the decades, the growth effects shrink dramatically and might even go negative: “Combined with reduced labor supply due to increasing tax rates on labor, the upward pressure on interest rates is projected to partially or wholly offset investments incentives by the end of the third decade,” JCT concludes.

JCT didn’t attempt to estimate how these growth effects will change how the law’s benefits are distributed. It finds that the reduction in individual tax rates will increase employment levels by about 0.6 percent in the short run, but that this effect would decline as the individual cuts are phased out.

Keep in mind that this is one economic projection among many. On the optimistic side, the right-leaning Tax Foundation finds that the bill would increase GDP by 2.7 percent over the long run, and increase middle-class incomes by 5 percent through supercharged growth; even they, however, find that the bill increases the deficit by more than $500 billion over 10 years despite those effects.

Nonpartisan modelers find much weaker effects. The broadly respected Penn Wharton Budget Model, which is overseen by UPenn economist and Bush administration veteran Kent Smetters, finds that GDP in 2027 would be 0.3 to 0.8 percent greater, and by 2040 would be between 0.2 percent lower and 0.5 percent greater. Basically, they find that the JCT’s estimate is an optimistic best-case scenario.

A poll by UChicago’s business school of America’s most eminent academic economists found that only one, Stanford’s Darrell Duffie, agreed that “If the US enacts a tax bill similar to those currently moving through the House and Senate — and assuming no other changes in tax or spending policy — US GDP will be substantially higher a decade from now than under the status quo.” Twenty-two economists, including Nobel laureates Angus Deaton and Richard Thaler, disagreed. Even Duffie added the caveat that “Whether the overall tax plan is distributionally fair is another matter.”

The White House is likely to push back strongly on the JCT numbers. “There is certain things we’ll agree with Joint Tax, there’s certain things we won’t necessarily agree with Joint Tax and we’ll explain the differences,” Mnuchin has said. He claims that analysis by the Treasury Department will confirm this.

But according to Alan Rappeport at the New York Times, that Treasury analysis purportedly showing the plan pays for itself does not exist. Mnuchin is making it up. In the meantime, we’re left with the judgments of reputable forecasters, who all say the bill will cost at least $500 billion and probably in excess of $1 trillion.