Pete Buttigieg, mayor of South Bend and 2020 Democratic presidential candidate, speaks with a member of the media in the spin room following the Democratic presidential debate in Los Angeles, California, U.S., on Thursday, Dec. 19, 2019.

Democratic presidential contender Mayor Pete Buttigieg was put on the defensive over a fundraiser he hosted inside a Napa Valley "wine cave" earlier this month, after rivals Sens. Elizabeth Warren and Bernie Sanders seized on the event during Thursday night's Democratic debate to criticize his high-dollar fundraising practices.

"We made the decision many years ago that rich people in smoke-filled rooms should not pick the next president of the United States. Billionaires in wine caves should not pick the next president of the United States," Warren fired off at the Los Angeles forum.

Surrogates for Sanders, a Vermont progressive, donned shirts displaying a URL that referenced the wine cave and linked to a Sanders donation page.

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Buttigieg defended his fundraising practices, arguing that he was the only Democrat on stage who was not a millionaire or billionaire, and rejecting what he called Warren's "purity" test.

"Senator, your net worth is 100 times mine," he said to Warren. "We need the support from everybody who is committed to helping us defeat Donald Trump."

Businessman Andrew Yang, who generally avoids criticizing his rivals, noted at one point in the debate that if Americans had more disposable income, candidates wouldn't have to "shake the money tree in the wine cave."

Read more: Here are the top moments from the sixth Democratic debate in Los Angeles

Buttigieg has proven to be an effective fundraiser despite his little experience on the national stage before his presidential bid. He raised more than $19 million during the last reported quarter. Warren raised almost $25 million, while Sanders raised $25 million.

A Buttigieg spokesperson said that 98% of the campaign's donations are under $200 and that the average contribution last quarter was $32. The spokesperson said Buttigieg will pursue campaign finance reforms as president, such as pushing to overturn the 2010 Supreme Court decision Citizens United and creating a small-dollar public financing system.