WASHINGTON (AFP) - As the joint flagbearers of progressive politics, Bernie Sanders and Elizabeth Warren rebuffed any suggestion their friendship was under threat ever since declaring their rival candidacies for the US presidency.
But that veneer of mutual respect has been shattered by a hot mic moment which caught the pair accusing each other of lying, dismaying leftwing supporters who fear the main beneficiaries will be the moderate contenders for the Democratic Party's nomination.
The first signs of tensions emerged earlier in the week with reports that Sanders told Warren in a private meeting in December 2018, as they contemplated presidential runs, that he did not believe a woman could win a presidential election.
Sanders, 78, called the reports "ludicrous", and flat-out denied uttering such a statement when the two of them appeared on stage at a televised debate in Iowa on Tuesday night (Jan 14).
Warren persisted, saying Sanders indeed made the comment, but the 70-year-old said she was "not here to try to fight with Bernie."
After the debate, a visibly irritated Warren refused to shake hands with Sanders, and the pair were seen in a heated verbal exchange, before they walked to opposite sides of the stage.
CNN, which aired the debate, eventually produced the audio, and it was explosive.
"I think you called me a liar on national TV," Warren said.
"You know, let's not do it right now. If you want to have that discussion, we'll have that discussion," Sanders said, as Warren replied, "Anytime."
"You called me a liar," Sanders continued. "You told me - all right, let's not do it now."
The debate itself was civil enough. But the explosive aftermath has dominated the Democratic primary coverage as the 12 candidates in the race - including former vice-president Joe Biden, the national frontrunner - gear up for the Iowa caucuses on Feb 3 which officially begins the presidential primary season.
CAN WOMEN WIN?
On Thursday, the two found themselves in close quarters again, as the impeachment trial of Trump ramped up in the Senate.
The collapse of the progressive esprit de corps has dismayed their political bedfellows, including filmmaker Michael Moore who has endorsed Sanders for president.
"Why Elizabeth chose to stick a knife in Bernie's back is beyond me," Moore tweeted on Wednesday, before CNN released the post-debate audio.
"At a time when job #1 is to remove Trump, how did this help?" Indeed, it is not helping, according to a network of 18 progressive groups who quickly called for unity.
"We pledge to focus our fight for the nomination against candidates supported by the corporate wing, instead of fighting each other," the network, led by Democracy for America, said in a statement.
"When progressives fight each other, the establishment wins," the group Justice Democrats added in a tweet, in a veiled swipe at Biden who by all accounts is the establishment candidate.
The clash has revived the ghost of the bitter fight between Sanders and Hillary Clinton in 2016.
The former first lady defeated Sanders in a marathon Democratic primary battle, only to lose to Donald Trump in November despite winning the popular vote.
After Tuesday's Warren-Sanders fracas, supporters on both sides vented their fury, prompting Sanders's wife Jane Sanders to personally appeal for a ceasefire.
"Forget those who are trying to divide us, focus on building the progressive movement & be positive to succeed!" she tweeted, adding the hashtag #MenWomenUnited.
The unexpectedly fierce spat has shone a spotlight on uncomfortable questions: can a woman win the presidency against Trump in 2020, and does sexism persist in American politics in the aftermath of the #MeToo movement?
Senator Amy Klobuchar, the only other woman on the debate stage on Tuesday, has also fiercely rejected the idea that a female candidacy is doomed to failure while acknowledging that resistance to such an idea persists.
"We have to grapple with the fact that some people think a woman can't win... I have heard about it from our own people (and) I've noticed it," Klobuchar told Politico.