The final Democratic presidential debate before primary voting gets underway began with a sharp focus on wars in the Middle East and trade, but wasn’t long before a dispute between progressive frontrunners brough an uncomfortable debate back to the fore:

Can a woman run for president and win?

Sen. Bernie Sanders of Vermont denied multiple reports that he had told Sen. Elizabeth Warren of Massachusetts in a private 2018 meeting that he didn’t believe that a woman could defeat President Donald Trump in a general election. Warren confirmed the conversation, hours after it was first reported by CNN.

“As a matter of fact, I didn’t say it,” Sanders said, one lectern over from Warren. “Anybody knows me knows that it’s incomprehensible that I think that a woman couldn’t be president of the United States.”

Warren was not asked directly whether Sanders was lying, but responded that when Sanders said that he didn’t feel that a woman could win the White House in 2020, she was succinct:

“I disagreed,” Warren said. “Bernie is my friend, and I’m not here to try to fight with Bernie… the question has been raised, and it’s time for us to attack it head-on.”

Warren pointed out that of the six candidates assembled onstage, the men had lost a collective ten elections—and that the only potential nominees in Des Moines that night who have won every election in which they have competed were herself and Sen. Amy Klobuchar of Minnesota.

“And,” Warren added, “the only person on this stage who has beaten a Republican incumbent any time in the past 30 years is me.”

(Sanders would later quibble that he defeated an incumbent Republican in 1990, leading to an awkward mini-debate about whether 1990 counts as “30 years ago.”)

The real danger for the party, Warren said, was not that a woman would face stiff headwinds from sexist attacks launched by the president, but that the Democratic Party would select a candidate “who can’t pull our party together.”

“We need a candidate who will excite all parts of the Dmeocratic Party, bring everyone in, and give everyone a Democrat to believe in,” Warren said, turning the electability question on its head. “Since Donald Trump was elected, women candidates have outperformed men candidates in competitive races.”

“Don’t deny that the question is there,” Warren concluded, noting that Presidents John F. Kennedy and Barack Obama faced questions about whether America was ready to elect a Catholic or a black man, respectively. “We changed America—that’s who we are.”

“ Anybody knows me knows that it’s incomprehensible that I think that a woman couldn’t be president of the United States. ” — Bernie Sanders

With less than three weeks until the Iowa caucuses, the exchange marked a de-escalation of this week’s hostilities, as Warren and Sanders’ no-contact campaign policy frayed over the course of an acrimonious back-and-forth over comments the Vermont senator made about Warren’s electability.

On the eve of Tuesday night’s debate, Warren had confirmed multiple news reports that Sanders had told her during that meeting that he didn’t believe that a woman could win against President Donald Trump in 2020, a claim that Sanders had called “ludicrous.”

“I thought a woman could win; he disagreed,” Warren wrote in a statement addressing the comments, which had been attributed to aides who had spoken with the Massachusetts senator after that meeting. “I have no interest in discussing this private meeting any further because Bernie and I have far more in common than our differences on punditry.”

Reports of the conversation came days after Politico reported that Sanders’ campaign had instructed volunteers to tell potential voters that Warren’s appeal was limited to “highly-educated, more affluent people,” a tactic that Warren called “factionalism” that threatens not only to tear apart the progressive wing of the Democratic Party, but could help Trump’s re-election bid.

“Democrats need to unite our party,” Warren told reporters during a campaign stop in Iowa. “I hope Bernie reconsiders and turns his campaign in a different direction.”

Tensions between candidates occupying similar ideological lanes have risen in recent weeks as other candidates have dropped out of the race, and as polling among the top tier has narrowed in Iowa—according to RealClearPolitics’ polling average, less than three points separate Warren and Sanders among likely caucus-goers, with Biden and Buttigieg, also neck-and-neck, a mere few points ahead.

Compared to exchanges between other 2020 Democratic hopefuls, the sniping between Warren and Sanders was borderline quaint, and the question of whether a female nominee might face stiffer headwinds against Trump was raised just last week by Biden. But for progressive Democrats who have grown accustomed to their unofficial non-aggression pact, even a game of bumper cars between the pair had some fearful of a two-car pileup.

“Most voters don't care to see Bernie's attack on Warren carry onto the debate stage,” Adam Green, co-founder of the Progressive Change Campaign Committee, which is backing Warren’s candidacy, told The Daily Beast in a statement. “But his attack underscores Warren's closing argument—that her inspiring message plus years of building coalitions instead of dividing people gives her a unique ability to unify Democratic voters and win swing voters to defeat Trump.”

Even Sanders’ backers appeared to push for lowering the temperature, particularly on the sensitive question of whether a woman can beat Trump in a general election, still a sore subject among Democrats following Hillary Clinton’s shock 2016 loss.

“I think with that whole situation, I kind of defer to both of their campaigns,” Rep. Alexandria Ocasio-Cortez, one of Sanders’ highest-profile endorsers and surrogates, told reporters on Capitol Hill on Monday. “But, you know, I think it’s also important to say that of course women can win.”

Before questions of a breakdown in relations between Warren and Sanders hijacked the pre-debate political sphere, both candidates had been signalling totally different targets in the final debate before the primary process officially begins. After heightened tensions between the United States and Iran put foreign policy at the forefront of voters’ minds, Sanders was publicly indicating that frontrunner Biden’s past support for the invasion of Iraq would be high on his to-do list. Warren, meanwhile, was preparing to drop a new plan outlining her proposals for cancelling student loan debt.

Even the temporary hostilities between the two darlings of the progressive movement, however, was enough to have conservatives excited at the prospect of Democratic infighting—including Trump himself.

Speaking at a campaign rally in Milwaukee, Wisconsin, moments before the Democratic debate began on Tuesday night, Trump weighed in on the electability controversy, declaring that “I don’t believe he said it”—although he did call Sanders “a nasty guy.”

“She said that Bernie said a woman can’t win,” Trump said. “I don’t believe that Bernie said that. I really don’t. It’s not the kind of a thing he’d say.”