Former treasury secretary Larry Summers calls it "grotesque" and "the supply-side economics of our time." It brings to mind other "free lunches" — as Summers also labels it — conceived by fringe thinkers and demagogic politicians of the 19th and early 20th centuries: Henry George's Single Tax; Huey Long's "Share Our Wealth" program; the Townsend Plan, in which all U.S. citizens at least sixty years of age would receive monthly government checks of up to $200 (financed by a national sales tax), every dime of which had to be spent by the next month; this, in turn, would stimulate and revitalize the depressed, 1930s economy.
The right's supply-side, trickle-down, Laffer-curve, voodoo economics has outlived them all, however; the concept still contaminates the minds of otherwise relatively normal people. At its most familiar level of economic incoherence, supply-side theory reasons that if some tax cuts can actually increase government revenue, then massive tax cuts, repeatedly applied, could raise massive amounts of revenue (or, at minimum, pay for themselves). From there, the lure of reductio ad absurdum seductively lulls not far away: eliminate all 
taxes and the federal government would be positively rolling in cash.
Aggrieved that it lacks its own grandiose economic absurdity, some progressive pols are being drawn to what's known as Modern Monetary Theory (MMT) — or, as Summers dubs it, the "grotesque," "supply-side economics of our time." From Yahoo! Finance: "The policy has been gaining traction on Capitol Hill as the likes of New York Democrat Alexandria Ocasio-Cortez and Vermont Senator Bernie Sanders fit the MMT framework with ambitious fiscal policies like the Green New Deal and universal healthcare."
In brief, MMT is attractive to those who wish to embrace hugely expensive government programs because government debt, as MMT sees it, is no longer an insurmountable inhibitor (much as supply-siding Republicans can overlook stifling debt whenever yet another round of tax cuts is due). Modern monetary theorists contend that "a country borrowing in its own currency can finance fiscal stimulus by printing money." Now you may suspect this would be hyperinflationary, and you would be correct. MMTers, however, counter that their theory "would allow the government to control inflation through tax policy. Instead of asking the Fed to stabilize prices through monetary policy, the government could raise taxes when prices get too high and cut taxes when prices get too low."
In actual practice? MMT hasn't worked out all that smoothly. Summers notes that among other problems — e.g., the government must pay "interest on any new money it creates" — in many developing countries ruinous hyperinflation was indeed the outcome (still is in, say, Venezuela). And the modern economies of France, Germany, Britain and Italy all experimented with "excessive reliance on inflationary finance," only to "reverse course" once unsustainability ensued.
Still, what I find most troubling about MMT and its progressive adherents isn't the bad economics. Rather, it's the abominable politics. The theory is the hallucinatory humbug of supply-sided thinking; it's designed to attract votes — not serious analysis or academic scrutiny.
Modern Monetary Theory is a way to bamboozle that segment of the electorate that very much wants, for instance, a Green New Deal, but does not want to ponder how it's to be paid for. The litany of Sandersesque free stuff would grow (in fact it already has) well past the Greenery: free healthcare, free preschool, free college, free child care, free whatever your progressive mind can conjure on the road to Utopia. Its promises would be a sham — a political swindle every bit as shameful, as cynical, as the right's promise of a virtually untaxed society with, nonetheless, all the goodies.
Do we really want both major parties steeped in the goof juice of rank demagoguery?