Liberal wonks often proudly say that their policy proposals are far more likely to be based on evidence than the proposals of conservatives. They scoff at the notion that big tax cuts can pay for themselves, or that loosening federal regulations on business could send economic growth through the roof. They have criticized the tax proposals of every Republican presidential candidate as budget-busting giveaways to the rich that rely on wild assumptions about the economy's growth prospects.

So it was notable that a group of left-leaning wonks -- including several former top economic advisers to Democratic presidents -- went hard at Bernie Sanders on Wednesday, attacking him for projecting that his proposals would deliver the type of enormous economic benefits that not even Republicans have dared to suggest their plans would accomplish.

Sanders' campaign has embraced projections that suggest, under his policies, middle class incomes would soar and economic growth would reach as high at 5.3 percent a year. That growth would, in turn, bring in more than enough tax revenues to fund his domestic spending priorities, including universal health care, free college, infrastructure and other projects. The calculations were done by a University of Massachusetts economist, Gerald Friedman.

"No credible economic research" supports those estimates, the former chairs of the Council of Economic Advisers wrote in a letter to Sanders. "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 signatories included Obama's former CEA chair -- Alan Krueger, Austan Goolsbee and Christina Romer -- and one of President Bill Clinton's, Laura D'Andrea Tyson.

Others were equally scathing, including New York Times columnist and Nobel laureate Paul Krugman and Brad DeLong, a University of California economist who served in Bill Clinton's administration.

"Arguing that Bernie Sanders's policies are likely to produce a 5.3%/year real GDP growth rate is not just wrong--not just likely to read to false conclusions about the likely impacts of the policies--but further opens the gates of hell for the likes of (conservative economists) Arthur Laffer and John Cochrane to dance around and get their garbage into the press," DeLong wrote in a blog post.

Sanders supporters dispute that.

"A democracy is not a graduate seminar," the blogger Steve Randy Waldman wrote this week, before engaging in a long Twitter debate on the merits of the CEA chairs' letter. "A democratic polity does not elect a technocrat-in-chief, but politicians whose role is to define priorities that must later be translated into well-crafted policy details."

This fight, though, could be bigger than wonks - if doubts about whether Sanders' proposals are realistic seeps into the minds of Democratic voters who have flocked to support Sanders.

The irony is, Sanders used to question whether sky high growth estimates were worth their talking points. Last summer, as he was just gaining steam, he made a point of saying economic growth should not be an end in itself, especially if all its benefits were going to the rich.

“Unchecked growth – especially when 99 percent of all new income goes to the top 1 percent – is absurd,” he said. “Where we’ve got to move is not growth for the sake of growth, but we’ve got to move to a society that provides a high quality of life for all of our people."

His plans now promise both.