The so-called bond king Jeffrey Gundlach is not shy when it come to rebuking the increasingly popular theory backed by progressives — the Modern Monetary Theory.

"MMT is a crackpot idea... sounds good for a first grader," the founder and chief executive officer of Doubleline Capital said in an investor webcast on Tuesday. He said the theory is "complete nonsense" being used to justify a socialist program.

The notion behind MMT is that as long as the Federal Reserve can keep interest rates low without sparking inflation, the national debt and budget deficit won't be an issue. MMT has been espoused by politicians including Rep. Alexandria Ocasio-Cortez, D-N.Y., and Democratic presidential candidate Sen. Bernie Sanders of Vermont.

Gundlach added that the "ridiculous" MMT is a way of monetizing and could lead to "a significant boycott of long-term bonds"

Fed Chairman Jerome Powell recently said the economic theory is unworkable.

"The idea that deficits don't matter for countries that can borrow in their own currency I think is just wrong," Powell said during congressional testimony in the Senate.