A couple months ago, one of my husband’s former colleagues from a progressive digital strategy firm popped up on his Facebook page to castigate him for supporting Hillary Clinton. “Matt, do you remember the exact date when you gave up?” the man wrote. “Was it when Obama turned out to be a damn conservative? Or were you never 100% behind this progressive thing to begin with? Tired of losing, so pick a candidate who, if she loses, it won’t really matter that much? I think it’s the last one. Sellout.”

That was the moment I realized that the Democratic primary, while incredibly high-minded compared with the Republican one, is creating lasting interpersonal enmity. On Saturday, Peter Wehner wrote in the New York Times about conservative friendships fraying in the age of Trump, describing people for whom “differences over the Trump candidacy have caused such a loss of respect that they feared their friendships would not survive, and that even if they did, they would never be the same.” I wish I could feel schadenfreude, but the same thing is happening among some committed progressives. Even now, with the primary season limping toward its foregone conclusion, collegial disagreement has given way to hostile incredulity, as people wonder how those who they thought saw the world in the same way could be so utterly, bafflingly wrong.

A necessary disclaimer—evidence for this is entirely anecdotal. The people who came to hate each other over the Democratic primary are a small, unrepresentative group of political obsessives. Most people never talk about politics online; in a 2012 Pew Research Center study, 84 percent of social media users said they’d “posted little or nothing related to politics in their recent status updates, comments, and links.” Like those Wehner writes about, people who’ve spoken to me about damaged relationships either work in liberal politics or are serious activists. They are part of a fairly minuscule subculture.

Among this little group, however, it’s easy to find people whose ties are being tested. “It has been an eye-opening and heartbreaking election cycle that has revealed some ugly truths about ‘progressive bros’ in my circle that will take some time for me to digest,” says Maryna Hrushetska, a 47-year-old curator and art adviser in Los Angeles who supports Clinton.* In the past, Hrushetska tells me, she’s worked on behalf of Palestinian rights and the environment, and she’s been shocked to see men she knows through those movements repeating sexist anti-Clinton slurs.

“The conflict is very unpleasant, but it’s also clarifying,” she says. “I believe there was a misogyny that existed below the surface among progressive men, thinking that because they’re environmentalists and drive Priuses and support the Palestinians, they are unassailable.”

On the other side is Khaldoun Khelil, a 39-year-old of Palestinian-Algerian descent who is appalled to see some of his female friends overlooking Clinton’s awful rhetoric on Palestine.* “I’m a passionate supporter of women’s rights and other progressive ideals, but when I ask for the same support from them to stand behind me and Palestinians—suddenly I’m a Bernie Bro,” he says. Khelil feels personally wounded by the silence of his Clinton-supporting friends in the face of their candidate’s lopsided pro-Israel rhetoric. “It just turned my stomach,” he told me. “I think the bad feelings will persist. It showed me that I’m kind of a lower peer.”

Talking to people on both sides of the divide, I heard similar sentiments over and over. People thought their friendships were built on a shared worldview. They thought their friends respected their experiences, their judgments, and their identities. But the primary has revealed opposing priorities and, fundamentally, different apprehensions of reality. “I feel like I’m living in the Twilight Zone,” says Katie Halper, a writer, radio host, and outspoken Sanders supporter. To her, Clinton’s flaws are manifold and glaring, and watching fellow feminists deny them is driving her mad. “This is the first time I’ve ever felt gaslit in my life,” she adds.

The term gaslighting refers to a manipulative denial of reality; it derives from the 1944 thriller Gaslight, about a man trying to make his wife believe she’s going insane. In talking about the primary, several people used it to describe frustration with friends who refuse to acknowledge obvious truths. “Here’s the thing about the gaslighting,” Hrushetska says of the anti-Clinton men she knows. “You’re trying to explain to them: ‘Her experience mirrors my experience of being a woman that’s trying to achieve something. Can you please listen to me?’ And they’re saying, ‘No, no, no, that does not exist, you’re using that as an excuse.’ ”

As with so many modern relational ills, much of the problem lies with the internet. It’s not just that people tend to be more obnoxious online than they are in person. The primary is revealing rifts among people who are used to assuming that their friends agree with them. Ordinarily, social media users who talk about politics congregate in polarized communities. Particularly on Facebook, they expect to revel in a shared sensibility, not to argue. “We find that our participants who perceive more friends as holding viewpoints different to their own engage less on Facebook than those with more similarity in their network,” says a 2014 Georgia Institute of Technology study about politics and online relationships. (The italics are in the original.) This primary—the first Democratic one since social media has become ubiquitous—has shattered the illusion of bien pensant unity.

“The first time I posted something critical of Sanders, the starting assumption was that I was somehow for greed or corruption or endless war,” says Mark Hershberger, 40, who once considered himself a leftist but now identifies as “pragmatic left-of-center.” “It’s hard to forget that someone implied that you’re a horrible person for not supporting their preferred candidate.”

Online political differences are particularly likely to poison relationships that aren’t all that deep to begin with. Before social media, we might not have known much about the opinions of friendly acquaintances. Now we’re confronted with them every day. “These experiences sometimes made our participants change their opinion about the friends,” the Georgia Institute of Technology study said about political conflict among the loosely connected. “Other times, it resulted in questioning the relationship and ultimately disassociating from the friend.”

Some of the current animosity among progressives could dissipate when the primary finally ends. That’s what happened in 2008, when tensions were at least as high as they are now. The ideological divide between Sanders and Clinton, however, is much wider than that between Clinton and Barack Obama. The 2008 primary was a battle over representation, raw because it pit the first female candidate with a legitimate shot at the presidency against the first black one. There was not, however, much of a gap between what Clinton and Obama hoped to accomplish in office.

This year is different. It’s a split between liberalism and the left, between those who seek greater representation within the existing system and those who would replace it entirely. Liberals can’t understand why those to their left refuse to recognize that incremental progress is better than none, particularly given the intolerable danger of the modern GOP. Leftists are increasingly convinced that liberals, ever eager to compromise, aspire to nothing beyond a more diverse ruling class and are thus obstacles to revolution.

“When I look back at the failures of the last 40 years, I don’t see it as the fault of Republicans, because they’re doing their job to advance their agenda,” says Angie Aker, a 37-year-old web writer and progressive activist in Kenosha, Wisconsin. “They’ve done a great job of it largely because the Democratic Party has let them.”

When I first spoke to Aker last week, she was unapologetic about alienating some of her friends with her ceaseless attacks on Clinton. “Relationships have cooled or fizzled because of my unwillingness to temper what I say about turning a blind eye to Hillary’s war hawkishness,” she said. On Tuesday night, I emailed her to see if she saw any prospect of interpersonal tensions easing as the primary winds down.

“I’m not going to refuse to do business with Hillary supporters or start fights with them at our friends’ bridal showers, but neither will I ever forget that when they had a chance to vote for and support a truly progressive future for people worse off than them, they decided a neoliberal feminist-in-name-only getting her turn was more important,” she replied. “It will color the way I see them from here on out, as I’m sure the force with which I’ve spoken against their views will color how they see me.” With that last part, at least, Clinton supporters will surely agree.

Read more Slate coverage of the Democratic primary.

*Correction, April 28, 2016: This article originally misspelled Los Angeles. (Return.) It also misidentified Khaldoun Khelil as Khelil Khaldoun. (Return.)