You might think a cut in taxes on investment would increase returns to investors in the long run. But the Tax Foundation’s model says that isn’t so — instead, it assumes investment would rise as much as was necessary to bid down pretax returns so that aftertax returns were unchanged. For example, automakers would pay a lower tax rate, but they’d make more cars, flooding the market until profit margins fell enough to fully offset the benefits of the tax cut. This assumption is how the Tax Foundation reaches the conclusion that Rubio-Lee would drive an enormous increase in investment.

This assumption led various economists to invoke the names of small islands.

“That’s true for the Netherlands Antilles, it’s not true for us,” said Doug Holtz-Eakin, the former head of the Congressional Budget Office who was John McCain’s top economic adviser during his 2008 campaign.

“It’s a model that might be appropriate for Bermuda,” Mr. Kotlikoff said.

In a very small, very open economy, the Tax Foundation might be right: Cuts in investment taxes would drive a flood of foreign capital, producing a huge percentage increase in investment. But the United States is simply too big for that to work. The U.S. economy also is not perfectly open; for example, we have some restrictions on trade. Therefore, estimates of the amount of investment created by investment tax cuts should be more modest. Economists also criticized the Tax Foundation model for assuming all that new investment would fall into place very rapidly, and for failing to address economic effects from spending cuts or increased borrowing that the tax cuts would require in their first years.

To be clear, Senator Rubio and Senator Lee have not claimed their tax plan would pay for itself. Mr. Rubio was quoted in Bloomberg as saying, “I’ve never believed that tax reform by itself should pay for itself.” While the Congressional Budget Office has not used dynamic scoring as the basis for official cost estimates in the past, it has provided informational estimates of major fiscal proposals’ effects on the economy, showing considerably more modest effects than the Tax Foundation finds, and with a significant range of uncertainty around those estimates.

For example, to estimate how tax cuts affect the economy, you need to estimate how much more people will work if you cut their taxes. The C.B.O.’s “central” estimate of this effect is about two-thirds what the Tax Foundation assumes, but it provides a range of possible effects, from close to zero to approximately as large as the Tax Foundation says. While some answers about how taxes affect the economy are wrong (as discussed above, the Tax Foundation’s estimates of taxes’ effects on capital are well outside the consensus range), there is no single answer about any parameter that is known to be right.

“I worry that, if they decide one way to do a dynamic score, it will seem to some people that has settled the debate over the likely economic effect,” Joel Slemrod, a public finance economist at the University of Michigan, said of the Congressional Budget Office. “You can’t say anything definitive.”