By Paul Krugman – New York Times.

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.

And the analysis is really something. The Republican candidates have been widely and rightly mocked for their escalating claims that they can achieve incredible economic growth, starting with Jeb Bush’s promise to double growth to 4 percent and heading up from there. But Mr. Friedman outdoes the G.O.P. by claiming that the Sanders plan would produce 5.3 percent growth a year over the next decade.

Even more telling, I’d argue, is Mr. Friedman’s jobs projection, which has the employed share of American adults soaring all the way back to what it was in 2000. That may sound possible — until you remember that by 2026 more than a quarter of U.S. adults over 20 will be 65 and older, compared with 17 percent in 2000.

Sorry, but there’s just no way to justify this stuff. For wonks like me, it is, frankly, horrifying.

Still, these are numbers on a program that Mr. Sanders, even if he made it to the White House, would have little chance of enacting. So do they matter?

Unfortunately, the answer is yes, for several reasons.

One is that, as the economists warn, fuzzy math from the left would make it impossible to effectively criticize conservative voodoo.

Beyond that, this controversy is an indication of a campaign, and perhaps a candidate, not ready for prime time. These claims for the Sanders program aren’t just implausible, they’re embarrassing to anyone remotely familiar with economic history (which says that raising long-run growth is very hard) and changing demography. They should have set alarm bells ringing, but obviously didn’t.

And there’s an even larger issue here: Good ideas don’t have to be sold with fairy dust.

Mr. Sanders is calling for a large expansion of the U.S. social safety net, which is something I would like to see, too. But the problem with such a move is that it would probably create many losers as well as winners — a substantial number of Americans, mainly in the upper middle class, who would end up paying more in additional taxes than they would gain in enhanced benefits.

By endorsing outlandish economic claims, the Sanders campaign is basically signaling that it doesn’t believe its program can be sold on the merits, that it has to invoke a growth miracle to minimize the downsides of its vision. It is, in effect, confirming its critics’ worst suspicions.

What happens now? In the past, the Sanders campaign has responded to critiques by impugning the motives of the critics. But the authors of the critical letter that came out on Wednesday aren’t just important economists, they’re important figures in the progressive movement.

For example, Alan Krueger is one of the founders of modern research on minimum wages, which shows that moderate increases in the minimum don’t cause major job loss. Christina Romer was a strong advocate for stimulus during her time in the White House, and a major figure in the pushback against austerity in the years that followed.

The point is that if you dismiss the likes of Mr. Krueger or Ms. Romer as Hillary shills or compromised members of the “establishment,” you’re excommunicating most of the policy experts who should be your allies.

So Mr. Sanders really needs to crack down on his campaign’s instinct to lash out. More than that, he needs to disassociate himself from voodoo of the left — not just because of the political risks, but because getting real is or ought to be a core progressive value.