Sens. Elizabeth Warren and Bernie Sanders both took aim at South Bend, Indiana Mayor Pete Buttigieg's reliance on campaign donations from billionaires and corporate executives during the 2020 Democratic presidential debate Thursday night, charging that a candidate heavily funded by big money cannot be trusted to pursue necessary progressive change to America's deeply unequal political and economic systems.

"If you can't stand up and take the steps that are relatively easy... how can the American people believe you're going to stand up to the wealthy and well-connected when you're president and it's really hard?"

—Sen. Elizabeth Warren

"Billionaires in wine caves should not pick the next president of the United States," said Warren, referring to Buttigieg's now-infamous big money fundraiser in Napa Valley, California, where attendees were served bottles of wine that sell for as high as $900.

Buttigieg responded by criticizing the personal wealth of Warren and other Democratic candidates onstage and invoking an argument that—as The Week's Ryan Cooper noted in a column Friday—has frequently been used to justify the corruption of the American political system by corporate cash.

"We need the support from everybody who is committed to helping us defeat Donald Trump," Buttigieg said.

Warren replied with a question: "If you can't stand up and take the steps that are relatively easy, can't stand up to the wealthy and well-connected when it's relatively easy when you're a candidate, then how can the American people believe you're going to stand up to the wealthy and well-connected when you're president and it's really hard?"

Watch the exchange:

Elizabeth Warren and Pete Buttigieg tussled Thursday night over high-dollar fundraisers, zinging each other over wine caves, hypocrisy and personal wealth https://t.co/oKNHqL4gx9 pic.twitter.com/scTRLp0nmC — POLITICO (@politico) December 20, 2019

Cooper ripped Buttigieg's rebuttal to Warren as "risible nonsense," writing that the South Bend mayor is "following a well-trodden path of centrist Democratic sellouts before him."

"Buttigieg's message that he will take oligarch money while simultaneously attacking the universal benefits and high taxes oligarchs hate... speaks for itself," Cooper wrote. "He's been bought and sold."

Following the back-and-forth between Warren and Buttigieg, Sanders said he is proud to be "the only candidate up here that doesn't have any billionaire contributions."

"But you know what I do have? We have received more contributions from more individuals than any candidate in the history of the United States of America at this point in an election, averaging $18 a piece," Sanders added.

The Vermont senator went on to mock the "real competition going on up here" between former Vice President Joe Biden and Buttigieg over the number of billionaires who have donated to their campaigns.

"My good friend, Joe, and he is a good friend, he's received contributions from 44 billionaires. Pete, on the other hand, he's trailing... You only got 39 billionaires contributing," Sanders said to laughter from the audience. "So, Pete, we look forward to you. I know you're an energetic guy and a competitive guy to see if you can take on Joe on that issue."

Biden and Wall Street Pete are paid off by their billionaire donors and someone calling them out on the debate stage for it was long overdue: pic.twitter.com/BNHerkjc9A — jordan (@JordanUhl) December 20, 2019

But billionaire funding of elections, Sanders said, is no "laughing matter."

"This is why three people own more wealth than the bottom half," said Sanders. "This is why Amazon and other major corporations pay zero in federal taxes. We need to get money out of politics. We should run our campaigns on that basis."