America’s two big political parties are very different from each other, and one difference involves the willingness to indulge economic fantasies.

Republicans routinely engage in deep voodoo, making outlandish claims about the positive effects of tax cuts for the rich. Democrats tend to be cautious and careful about promising too much, as illustrated most recently by the way Obamacare, which conservatives insisted would be a budget-buster, actually ended up being significantly cheaper than projected.

But is all that about to change?

On Wednesday four former Democratic chairmen and chairwomen of the president’s Council of Economic Advisers — three who served under Barack Obama, one who served under Bill Clinton — released a stinging open letter to Bernie Sanders and Gerald Friedman, a University of Massachusetts professor who has been a major source of the Sanders campaign’s numbers. The economists called out the campaign for citing “extreme claims” by Mr. Friedman that “exceed even the most grandiose predictions by Republicans” and could “undermine the credibility of the progressive economic agenda.”

That’s harsh. But it’s harsh for a reason.

The claims the economists are talking about come from Mr. Friedman’s analysis of the Sanders economic program. The good news is that this isn’t the campaign’s official assessment; the bad news is that the Friedman analysis has been highly praised by campaign officials.