Bernie Sanders' promises about the economy are overly rosy and threaten the Democratic Party's credibility, former economic advisers to Presidents Obama and Clinton warned Wednesday.

In an open letter posted on a dedicated website, the economists told the Vermont senator and Democratic presidential candidate that "[m]aking 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 economists, comparing the vision of the future that Sanders has sketched out on the campaign trail to the over-optimistic scenarios Republicans have said would follow their tax plans, cautioned that "no credible economic research supports economic impacts of these magnitudes."

Specifically, the four Democratic advisers warned Sanders that the analyses prepared for his campaign proposals by economist Gerald Friedman are not reasonable. Friedman, with the University of Massachusetts, Amherst, has estimated that Sanders' proposals, such as instituting single-payer healthcare, spending $1 trillion on infrastructure, and raising the minimum wage to $15 an hour, would send unemployment plummeting and incomes soaring. Friedman projected that, under Sanders' plans, gross domestic product would surge to 5.3 percent annually, rather than the 2.1 percent forecasted by the Congressional Budget Office. Poverty would fall by two-thirds, while median incomes would increase by 60 percent over current projections.

"These claims undermine the credibility of the progressive economic agenda and make it that much more difficult to challenge the unrealistic claims made by Republican candidates," wrote the Democratic advisers.

Signing the letter were four former chairmen of the Council of Economic Advisers. They are Obama advisers Alan Krueger of Princeton, Austan Goolsbee of the University of Chicago and Christina Romer of the University of California, Berkeley. They were joined by Laura D'Andrea Tyson, a Berkeley Haas professor who advised Clinton in the 1990s.