In the days since the Working Families Party endorsed Senator Elizabeth Warren for President on Monday, staffers at the progressive group have received hundreds of furious messages, including several using racial slurs against black staffers and at least one celebrating a junior employee’s rape.

Maurice Mitchell, the National Director of the Working Families Party (WFP) and an organizer with the Movement for Black Lives, received messages calling him a “half man” and “Uncle Tom,” and telling him to “go back to his slave masters,” according to emails and tweets reviewed by TIME.

WFP organizer Ember Ollom, a rape survivor who has tweeted about her assault, received a message that said: “We were raped by this process, so I’m happy it happened to you.”

When Ollom tweeted about the harassment, more people responded to tell her she was lying.

Staffers also received messages and tweets that vowed to “hunt all WFP down” and said “may you all get cancer.” One staffer received a threat that someone would find where she lived.

The backlash began after the party, which has an organizing presence in 20 states and is on the ballot in three, endorsed Warren’s 2020 bid on Monday after endorsing Sanders in 2016. The endorsement, one of the first from a progressive group, was seen a triumph for Warren and a setback for Sanders, who has praised the Working Families Party as an example of “my vision of Democratic socialism.”

In the days since, WFP staffers were inundated with messages calling them “liberal sellouts,” “corporate frauds,” and “just as corrupt as the DNC,” often accompanied with the hashtag #BernieorBust. It was not immediately clear how many of the messages were sent by real people, how many were sent by right-wing agitators, or how many were sent by bots.

Sanders himself has long refrained from personal attacks on his rivals or their supporters, and has been quick to decry harassment undertaken in his name. “This campaign condemns racist bulling and harassment of any kind, in any space” Sanders tweeted Thursday. “We are building a multiracial movement for justice—that’s how we win the White House.”

In response to requests from TIME, Sanders’ campaign declined to offer further comment.

“I do not personally think that Bernie Sanders would condone this behavior,” says Ollom. “It’s not all Bernie supporters, but people are doing this in his name. A lot of people have their picture as his profile picture. When it seems to come from a collection of people speaking on behalf of this candidate, it definitely looks bad.”

Many Sanders supporters expressed outrage at an endorsement process they viewed as too secretive. The WFP endorsement process—which was developed for the 2020 primary and was not used in 2016—gave equal weight to its National Committee (made up of activists, organizers and union leaders who had been elected to represent their respective constituencies, similar to party delegates) and its grassroots online membership (including non-paying party members and the general public on their email list.) The 2016 process had given the National Committee significantly more weight than the grassroots membership, and the new process was developed to equalize the input in the 2020 endorsement. The Party announced Monday that Warren received more than 60% of the vote through an endorsement process that gave equal weight to the two factions, but did not release vote breakdown.

“We think it’s counterproductive for supporters in two camps to engage like this,” says Mitchell. “Some people are disappointed with the outcome, so they’re raising questions about the process.”

The backlash to the endorsement hints at a turbulent mood in a progressive base that poses looming challenges for both Warren and Sanders. For Warren, the challenge will be to convince hardcore Sanders supporters that she is an equally progressive candidate. For Sanders, the challenge will be keeping some of his base from defecting.

“I think it’s important for people to realize that when they send these messages, the Working Families Party is not answering them, the people who work there are answering them,” says Ollom. “We’re people with feelings. This kind of division is only harming us. It’s not furthering any of our candidates. This kind of behavior doesn’t make me want to help them. They’re going to end up alienating undecided voters.”

Write to Charlotte Alter at charlotte.alter@time.com.