Bernie Sanders sought to swiftly fact check Elizabeth Warren’s debate-stage claim Tuesday night that she's the only candidate of the bunch who beat an incumbent Republican over the last 30 years.

Warren made that claim after Sanders denied an explosive report that he once told Warren that a woman couldn’t win the presidency. Warren swiped at the electability of her rivals, saying the men on stage lost 10 elections while the two women are undefeated.

Sanders reminded his colleague in the Senate that he defeated an incumbent Republican during his congressional run in 1990.

“30 years ago is 1990,” Sanders said.

Sanders, who at the time was mayor of Burlington, Vermont, defeated incumbent Republican Rep. Peter Smith to win the seat representing the state’s at-large congressional district.

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The moment was part of the tensest exchange between the Vermont senator and his colleague from Massachusetts after reports emerged that Sanders allegedly told Warren that a woman could not win a presidential race.

Sanders has denied that. But the Massachusetts senator said in a statement Monday that during their two-hour meeting two years ago to discuss the 2020 election, “among the topics that came up was what would happen if Democrats nominated a female candidate. I thought a woman could win; he disagreed.”

The dispute marked an extraordinary turning point in a Democratic primary that, with few exceptions, has been characterized by genial differences over domestic issues such as health care. Sanders and Warren, whose policies on issues ranging from health care to student loans are closely aligned, have notably backed off from criticizing one another in previous debates.

CNN first reported Sanders’ comment earlier Monday, based on the accounts of anonymous people with knowledge of the meeting. That drew a swift and strong denial from Sanders.

"I didn’t say it,” Sanders said during Tuesday’s debate. “This is what Donald Trump and maybe some in the media want.”

He added: "It’s incomprehensible to think of me saying a woman couldn’t become president of the United States. How could anybody in a million years believe a woman can’t be president of the United States.”

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Sanders aides have accused Warren’s campaign of leaking what they said was an inaccurate description of what was said during the meeting.

That prompted Warren’s statement hours later. Jeff Weaver, a senior adviser to Sanders, then seemed to try and defuse the situation, refusing to refute Warren's version and instead saying only on CNN on Monday night that “those conversations can sometimes get misconstrued.”

Still, the controversy is likely to revive anxiety among Democrats about whether — nearly four years after Hillary Clinton lost her White House bid — voters are willing to support another woman running for president. Such questions have dogged Warren and other female candidates throughout the 2020 campaign.

The Associated Press contributed to this report.