In an interview with Axios' Mike Allen for "Axios on HBO," economists Emmanuel Saez and Gabriel Zucman, who helped guide Democrat Sen. Elizabeth Warren's 2020 wealth tax plan, said America has entered a new Gilded Age.

The big picture: Warren's "ultra-millionaire tax" is one of the most progressive tax plans on the Democratic market. It aims to adjust rates so that the rich are not paying less, proportionately, in taxes than the wealthy. Opponents of the plan argue it would de-incentivize investments and stifle economic growth.

A paper released by Saez and Zucman in September analyzed how the plan would have affected the 15 wealthiest Americans' fortunes if it was in place since 1982. It found that subjects' net worth could have decreased by more than half.

What they're saying: "You look at the numbers and it's very similar to the Gilded Age... Taxation was very regressive during the Gilded Age," Zucman said.

Saez also argued America is facing new monopolies: "The equivalent of the Gilded Age today, are the Microsofts, the Apples, the Amazons — incredible companies that really transform the world, but you can see that they really monopolize things because everybody goes, you know, to the same provider and that dominates, entirely, the market."

Go deeper: Elizabeth Warren: What to know about the 2020 candidate's wealth tax proposal