WASHINGTON — For a year they offered almost nothing but praise for each other in public, he paying tribute to her populism, she initially adopting his health care plan, and each supporting the other against moderates in the Democratic presidential debates and rebuking reporters who sought to stoke any suppressed rivalry.

The alliance between Senators Elizabeth Warren and Bernie Sanders was an extension of their rapport in the Capitol, which was more about mutual interest than personal friendship, but it remained largely intact through 2019 despite highs and lows for each candidate — even as they competed for the same set of voters.

But that relationship has now ruptured in extraordinary fashion, over whether he told her in a private meeting in 2018 that a woman couldn’t defeat President Trump. The breach runs a real risk of damaging liberal hopes of ultimately uniting Democratic Party factions around a progressive nominee in 2020.

A sizable, loud swath of left-wing activists are now furious at Ms. Warren for criticizing and challenging Mr. Sanders over gender and for employing hardball campaign tactics this week. Meanwhile, some progressive women and backers of Ms. Warren are angry at Mr. Sanders for disputing her and for not denouncing the #NeverWarren campaign by some of his online supporters.

Both candidates and their advisers have come to believe that the other camp is fighting dirty, according to interviews with campaign officials and surrogates. They have not spoken since their postdebate confrontation in Iowa on Tuesday night, and many liberals are starting to feel a painful sense that they’ve been here before.

“We can’t relive 2016,” said Randi Weingarten, the head of the American Federation of Teachers, referring to the split between pro-Sanders liberals and other Democrats who wanted the party to unite behind his onetime rival, Hillary Clinton. “Misinformation miscreants will have a field day with this.”

Ill will between Sanders and Warren supporters is now exploding on social media, with angry memes, conspiracy theories and harassing commentary mostly aimed at Ms. Warren. Negative attacks and doubts about party unity are exactly what Democratic leaders do not want to happen in 2020, for fear of handing ammunition to Mr. Trump, as the heated contest between the Vermont senator and Mrs. Clinton did four years ago.