And the fact that the Hillary Clinton campaign, and its surrogates, are now deploying this line of attack against Vermont Senator Bernie Sanders is likely evidence of how worried they are about Sanders’ popularity among the Democratic electorate.

The latest example of this attack came Wednesday in an open letter written by four former Obama and Clinton administration economists that was addressed to Sanders and University of Massachusetts economist Gerald Friedman, who has recently made some extraordinary claims about what sort of economic growth would result from the implementation of the Sanders economic agenda. They write:

For many years, we have worked to make the Democratic Party the party of evidence-based economic policy. When Republicans have proposed large tax cuts for the wealthy and asserted that those tax cuts would pay for themselves, for example, we have shown that the economic facts do not support these fantastical claims….

We are concerned to see the Sanders campaign citing extreme claims by Gerald Friedman about the effect of Senator Sanders’s economic plan—claims that cannot be supported by the economic evidence. Friedman asserts that your plan will have huge beneficial impacts on growth rates, income and employment that exceed even the most grandiose predictions by Republicans about the impact of their tax cut proposals.

As much as we wish it were so, no credible economic research supports economic impacts of these magnitudes. Making such promises runs against our party’s best traditions of evidence-based policy making and undermines our reputation as the party of responsible arithmetic.

The authors of the letter—economists Alan Krueger of Princeton, Austan Goolsbee of the University of Chicago, and Christina Romer and Laura Tyson of the University of California—are complaining about an article published in CNNMoney that first reported the content of a research paper by Friedman which argued that Sanders’ policies could bring annual real GDP growth in America to 5.3%.

In the same article, Sanders policy director Warren Gunnels is quoted as calling Friedman’s paper “good work,” and defended the eye-popping estimates made by the University of Massachusetts economist.

So, what are the problem with Friedman’s analysis? Economist Paul Krugman took him to task on his blog for overestimating the potential of public policy to help increase the labor participation rate—the rate at which adults are in the workforce. “Even those of us who believe that there’s still significant slack in the US labor market are aware that much, probably most, of the decline in labor force participation since 1999 reflects an aging population,” Krugman writes. He continues:

The point is not that all of this is impossible, but it’s very unlikely—and these are numbers we would describe as deep voodoo if they came from a tax-cutting Republican.

Sanders needs to disassociate himself from this kind of fantasy economics right now. If his campaign responds instead by lashing out—well, a campaign that treats Alan Krueger, Christy Romer, and Laura Tyson as right-wing enemies is well on its way to making Donald Trump president.

In a phone interview with Fortune, Friedman says that none of the economists who signed the letter reached out to him to ask for a copy of the paper. “I don’t know if they read my report, quite honestly,” he says.

Though he won’t speculate why these economists felt the need to question his analysis, he says that there is a tendency in the economics community to doubt that government action can boost economic growth and to instead assume that trends like a declining participation rate are the result of an aging population, but that such assumptions are unproven.