DES MOINES — They clapped and cheered for Sen. Elizabeth Warren as if she was their chosen candidate, until they didn’t.

At a watch party for supporters of Sen. Bernie Sanders in Des Moines on Tuesday, the tide turned halfway through the debate when Warren (D-Mass.) was asked about the public feud that has dominated the campaign in the last 48 hours between her and Sanders (I-Vt.) over her claims that he told her in a private conversation last year that a female candidate couldn’t win the presidency.

The Sanders watch party, held in a private event space in downtown Des Moines, went silent as the dispute came up. Supporters cheered as Sanders denied saying that a woman couldn’t win, but when Warren doubled down, many in the room gasped in shock, stunned at the collapse of what they viewed an unspoken alliance between the two most liberal candidates in the race.

After that, many supporters groaned or even booed when Warren’s face popped up on the television screen for the rest of the night — an even more hostile reaction than the one given to other candidates such as former vice president Joe Biden and former South Bend, Ind., mayor Pete Buttigieg, candidates that are very unpopular in Bernie world.

“I just honestly can’t believe it,” said Renee Hessing, a 30-year-old substitute teacher from Des Moines. She caucused for Sanders in 2016 and is strongly supporting his candidacy again but had a soft spot for Warren, hopeful that at least one of the liberal candidates in the race could succeed.

Even though it was a “pipe dream,” she had dreamed of a possible Sanders-Warren ticket. “I love them both,” she said.

But Warren’s decision to double down on her claim was, in Hessing’s view, a “total misstep,” one she said would probably prove damaging among legions of Sanders supporters, who would see her action as a betrayal. The way Warren handled it, she said, seemed designed to prolong the drama. “It’s like a fight you have with your boyfriend, and you’re like, ‘I don’t want to talk about it anymore’ right after you say something offensive, and it just doesn’t stop,” she said. “The fight gets worse.”

While Sanders’s events in Iowa have grown increasingly diverse in age and gender in recent weeks, as undecided voters have taken a new look at his campaign, his watch party was packed with the kind of young voters that had fueled his insurgent campaign four years ago. Even though that race had grown exceedingly ugly and personal, no one here believed Sanders would say what Warren had accused him of — including Hessing.

“It’s so stupid: Of course a woman can win. Hillary Clinton won the popular vote by 3 million people,” she said. “I don’t believe he would say something like that.”

But she worried the dispute would have deeper ramifications, potentially dividing liberal voters between Sanders and Warren and ushering in a more centrist candidate that wouldn’t excite anybody on the left.

“This feels like a fight that Donald Trump wanted us to have, just to divide us so he can win,” she said. “And it might. It really might.”