Yet the moment is also serving to galvanize the Vermont senator’s legion of diehard supporters, who are quick to seize on any perceived slight to his campaign and see a dirty trick perpetrated against him just as polls show him taking the lead in Iowa.

“The fire of that 2016 campaign forged an incredibly strong base of support for Sanders like steel,” said Neil Sroka, activist with the progressive group Democracy for America, which is trying to broker a peace between the liberal favorites, worried that divided progressives will make it easier for establishment candidate Joe Biden to win.

“In that campaign in 2016 you had a lot of duplicity from Democratic insiders and we saw for better or worse in leaked emails from the party — concrete evidence — that insiders were against him,” Sroka said. “And that fuels them today.”

It’s that energy for Sanders that has put him in the lead, or within striking distance, in the first three early-voting states where organization and motivation are musts. Yet it’s also put him at risk in the run-up to the Feb. 3 caucuses in Iowa, where 57 percent of the electorate was female in 2016. The passion of some online Sanders’ backers — widely referred to as the Bernie Bros — have revived bad memories from 2016, when the campaign felt the need to have their supporters stop savaging Clinton staffers and female reporters, columnists and bloggers with misogynistic attacks.

After the Tuesday night debate, his supporters filled Twitter with emojis of snakes to refer to Warren and caused “#LyingLiz;” “#NeverWarren;” and “#WarrenisaSnake” to trend Wednesday on the social media platform.

The controversy could “absolutely” haunt Sanders now, said Adrienne Elrod, who was Clinton’s director of strategic communications.

“This is the danger of this issue coming up right now,” she said. “While Sanders has been outwardly supportive of women, many of us were — AND still are — frequent targets of the Bernie Bros, an online community that supports Sanders. Those negative, stomach-churning feelings have now been given a platform to come to life. This is not what Sanders and his campaign want to be talking about.”

Elrod added that the dust-up could “help Warren potentially siphon off some of Sanders female supporters at a crucial time right before the Iowa primary.”

Both the Warren and Sanders campaigns declined Wednesday to talk publicly about the controversy, which was given new legs after cameras captured a tense exchange between the two candidates after Tuesday’s debate, where Warren approached Sanders and appeared to rebuff a handshake he offered before speaking to him briefly.

President Trump added to Sanders’ woes in a Democratic primary by siding with him, saying he believed Sanders’ version of events.

The candidate’s wife, Jane O’Meara Sanders, sought to defuse the situation Wednesday.

“The @BernieSanders campaign is about unifying people across our country — not dividing them up by race, ethnicity or gender,” Sanders wrote. “Forget those who are trying to divide us, focus on building the progressive movement & be positive to succeed!#MenWomenUnited.”

In September, Sanders himself had to tweet he was opposed to “racist bullying and harassment” after some backers besieged the Working Families Party for endorsing Warren, whose advisers contend that marked the beginning of aggression against her from Sanders’ partisans.

That hostility increased after POLITICO reported last week that Sanders’ field staff was using unflattering talking points to describe Warren.

Warren’s account of the private 2018 one-on-one conversation between them appeared in the media not long afterwards.

Asked about it onstage during Tuesday’s debate, Sanders denied he made the remarks. When a CNN debate moderator then asked Warren for her version of events, framing the question in such a way that it accepted her version of events as true, some Sanders supporters insisted the fix was in. After all, CNN broke the story about the remarks just before hosting its debate, ensuring maximum impact.

Following the debate, Sanders’ campaign co-chair, Nina Turner, fumed to reporters about “faux feminism.” Online, other supporters called Warren a liar and brought up her misrepresentations about her Native American ancestry. And they pointed out that Sanders — who has publicly advocated for women to be president for years — has the highest percentage of women donors to his campaign of any Democrat.

Still others complained that Warren has never attacked Biden over abortion and his support of a widely derided bankruptcy bill, a longstanding point of contention between them. Instead, Sanders’ supporters note, Warren attacked Sanders in a potentially devastating way in a progressive primary.

A top Warren aide privately said there’s no backroom deal with Biden and accused the Sanders campaign of hypocritically dishing out campaign attacks and then whining when she hits back.

Karen Finney, a senior Clinton adviser, said she’s familiar with the reaction of Sanders supporters who rally when Sanders is attacked, especially considering how the topic that played out this week. “[T]here seems to be an element of sexism, grievance and Bernie is fighting for them,” she said.

Democratic women have reacted viscerally to the condemnation of Warren by Sanders backers because it reminds them so much of what happened four years ago, Finney said.

“This time they’re not going to let the Bernie Bros do what they did last time,” she said. “There’s resentment still. There’s a scar there. And the wound reopened.”

Alex Thompson contributed to this report.