Tuesday night’s Democratic debate defied my expectations and remained largely civil. I wasn’t thrilled that my first choice, Andrew Yang, had been excluded because of a dearth of early January polling, but the remaining candidates had a nuanced discussion of foreign policy and largely steered clear of personal attacks. The one notable exception to this broader trend of civility, however, was the messy onstage breakup of progressive candidates Bernie Sanders and Elizabeth Warren.

It’s clear by now that Senators Sanders (I-VT) and Warren (D-MA) are natural allies with well-established progressive bona fides. Warren was instrumental in the creation of the Consumer Financial Protection Bureau, which regulated banks in the aftermath of the 2007 collapse. Sanders’ progressive credentials stretch back decades; his 2016 campaign centered around an opposition to Hillary Clinton from the left. The two candidates worked together in the Senate, their campaign policies are remarkably similar and they have clearly drawn inspiration from one another in the past.

However, some of their respective supporters have always remained wary of one another. In June 2016, after Hillary Clinton won the California primary and secured the majority of delegates, Warren endorsed her over Sanders for the presidency. It’s worth noting that President Obama and Vice President Biden endorsed Clinton on the same day, but Sanders-supporting progressives quickly singled out Warren for criticism, arguing that even though her positions were closer to Sanders’, she had thrown him under the bus for political points. This tension died down after the election, but provided those with an axe to grind against Warren an arguably valid justification for doing so.

The essence of the current Sanders-Warren feud originates in differing accounts of a closed-door meeting that the two held in December 2018. Though the details remain unclear, it has been commonly reported that Sanders and Warren, both of whom planned to seek the Democratic nomination in 2020, made an informal agreement to avoid opposing one another in public. Since they compete for the same base of voters, for either to have any chance of clinching the nomination, one would eventually need the support of the other.

This agreement may have ended last week. During Tuesday night’s debate, Warren alleged that Sanders had told her privately during the December 2018 meeting that he did not believe a woman could win the 2020 election, a contention that Sanders vigorously denies. Though they did not discuss it further during the debate, the two of them were recorded on a hot mic afterwards accusing one another of lying. That seems to be the only possible conclusion; if there is a middle ground between their positions, it is a very small one.

I won’t try to make the judgement of who is telling the truth and who is not. I will observe, however, that whatever else one thinks about Sanders, he’s nothing if not ideologically consistent; for better or worse, he’s been advocating for the same collection of populist policies since he was mayor of Burlington, Vermont in the 1980s. There’s footage of him on YouTube encouraging a classroom of young girls to run for President one day. It’s certainly possible that Sanders believes that many Americans will not vote for a woman, but it seems a stretch for him to think that this would automatically disqualify Warren.

After their sniping on Tuesday night, neither Warren nor Sanders has commented on the incident, and both have since been drawn away from the campaign trail to serve as jurors on President Trump’s impeachment trial. The same cannot be said of their respective fanbases, which have turned on one another with surprising vitriol. This is especially true of Sanders’ adherents on social media, who have inundated Elizabeth Warren’s pages with insulting comments and the “snake” emoji. They have also targeted CNN, whose moderator Abby Phillip was widely ridiculed for asking Warren how she had reacted to the statement that Sanders had made without acknowledging that he denied making it.

Whatever one thinks of CNN and of Warren, though, the debacle reflects particularly badly on Sanders’ partisans on social media, whose smear campaign against Warren straddles the boundary between largely unfair criticism and overt misogyny. This has resurrected old fears that some of Sanders’ adherents — the “Bernie bros” — support him out of opposition to the women in the race, ironically providing evidence for Sanders’ supposed point.

For more mainstream liberals, it’s hard to see the feud as anything other than a needless division within the Democrats’ ranks. One anonymous Democratic operative described it as “the most idiotic thing [he’d] ever seen.” Indeed, it’s hard to see what Senators Sanders and Warren are thinking. Whether or not her allegation against him is true, the strategic calculus for both candidates remains the same as it was in December 2018 — they need to pick up the other’s base if the other drops out before they do. This isn’t likely to happen if the two candidates or their supporters are at each other’s throats about an off-the-cuff remark made over a year ago.

In that sense, the only real beneficiaries of the unraveling of the left will be the candidates that both Sanders and Warren hope to defeat — Biden, Buttigieg, Klobuchar, and eventually Trump. The two senators’ adherents ought to take note.