Bernie Sanders, the Vermont senator who has ignited a fire among progressives with his cry for political revolution ahead of tonight's caucus in Iowa, has largely resisted the urge to sling personal insults at opponent Hillary Clinton on the campaign trail — but the same cannot be said for a small but vocal group of his supporters, dubbed the Bernie Bros.

At first, the Bernie Bros were cute, even funny. The meme-friendly term was a tongue-in-cheek way to describe Sanders's fanbase, which skews, as Quartz describes it, "educated, politically leftist white men." The Bernie Bros are progressive. They are likeable and well-meaning. They love NPR, improv comedy, and Reddit. Yahoo reports that a group of Bernie superfans has mobilized a grassroots campaign called "Operation Iowa" to canvass for Sanders ahead of the Iowa caucuses. More than 160,000 people (men and women) have contributed to the Sanders campaign through online grassroots efforts, Mashable reports. Some of them look like this:

But recently, a dark side of the Bernie Bro has emerged. Women began noticing that a subset of Sanders supporters on the Internet were hateful to those who are critical of their candidate in any way, or who support Hillary Clinton. Journalist Sarah Jeong, a contributing editor at Vice Motherboard and who has written a book about online harassment and free speech on the Internet, had to set her account to private to block all the vitriol coming from trolls who claimed to support Sanders. Ironically, Jeong herself is a Sanders fan. Mother Jones co-editor Clara Jeffery flagged the online harassment of Jeong and the Bernie Bro phenomenon:

This content is imported from Twitter. You may be able to find the same content in another format, or you may be able to find more information, at their web site.

1/ Another week, another woman mercilessly harassed on @Twitter. This time it's @sarahjeong, who's had to lock her account. — Person Woman Man Camera TV (@ClaraJeffery) January 28, 2016

This content is imported from Twitter. You may be able to find the same content in another format, or you may be able to find more information, at their web site.

2/ This campaign of harassment began with some who self-identify as @SenSanders supporters. cc @nickconfessore pic.twitter.com/qwVJYWDnmU — Person Woman Man Camera TV (@ClaraJeffery) January 28, 2016

This content is imported from Twitter. You may be able to find the same content in another format, or you may be able to find more information, at their web site.

3/ @SenSanders @BernieSanders campaign has been telling folks to stop this kind of abuse. Here's @cascamike: pic.twitter.com/IYvlUZKpso — Person Woman Man Camera TV (@ClaraJeffery) January 28, 2016

This content is imported from Twitter. You may be able to find the same content in another format, or you may be able to find more information, at their web site.

4/ Now, are some harassers only pretending to be Bernie supporters. Probably. Is dogpiling restricted to one candidates' followers. No. but — Person Woman Man Camera TV (@ClaraJeffery) January 28, 2016

This content is imported from Twitter. You may be able to find the same content in another format, or you may be able to find more information, at their web site.

5/ This is not an isolated incident. Nor isolated to women. cc @elonjames — Person Woman Man Camera TV (@ClaraJeffery) January 28, 2016

This content is imported from Twitter. You may be able to find the same content in another format, or you may be able to find more information, at their web site.

6/ As for the people saying "she's only harassed b/c she complained of harassment without proof" I mean, listen to yourself, FFS. — Person Woman Man Camera TV (@ClaraJeffery) January 28, 2016

This content is imported from Twitter. You may be able to find the same content in another format, or you may be able to find more information, at their web site.

7/ As for those who think people complain about this are all secret Hillarybots, here: pic.twitter.com/CJGSKuVgGI — Person Woman Man Camera TV (@ClaraJeffery) January 28, 2016

This content is imported from Twitter. You may be able to find the same content in another format, or you may be able to find more information, at their web site.

8/ Finally, @Twitter @support @jack: Please give us better tools to help not make your platform a nightmare for >50% of population. — Person Woman Man Camera TV (@ClaraJeffery) January 28, 2016

Others have noticed the phenomenon too: One unnamed chief-of-staff for a lawmaker who endorses Clinton told Mashable, "The most obnoxious people are the Bernie supporters on social media."

"We've gone out of our way not to be critical of him ever," said the official. "But any time she posts anything on her political page about Hillary, we get all of these Bernie people just being ridiculous. And I've talked to staff for other members, and it's just exhausting."

Some male journalists, like Glenn Greenwald, believe that Bernie Bros are an invention by pro-Clinton journalists to distract from political issues and that this sort of online harassment is typical of life as an Internet writer or anyone who openly discusses political candidates, and has nothing to do with the Sanders campaign or his supporters.

But feminist writers, especially those who have grappled with the decision to support Clinton and have been targeted by this particular form of harassment, would beg to differ. The vitriol is even more shocking, perhaps, because it runs so counter to the values Sanders espouses. New York Magazine's Rebecca Traister wrote of the Bernie Bro phenomenon: "These loose, careless characterizations, sometimes made without specific reference to individuals or acknowledgment that Sanders fans and Clinton critics are far from all-male or all-white, may be annoying and unfair. But they exist because they describe a dynamic — sexist condescension and dismissal of feminist argument — that is happening online."

How can you recognize a Bernie Bro? He's usually part of a vocal minority of progressive men that shuts down or ridicules those who call Clinton anti-establishment because she is a woman, or who say anything critical about Sanders at all. This activity has been going on for some time — as reported by the Washington Post last September, some Sanders supporters had taken to attacking on Twitter those who questioned the candidate's record on civil rights.

The Bernie Bro is not just an Internet phenomenon either. In real life, the condescension and anger inherent in a bro perhaps takes the form of "mansplaining." Olivia O'Hea, a senior at Iowa's Drake University who first supported Sanders but switched to Clinton, spoke about her experiences with Bernie Bros in Time:

O'Hea ... noted that Sanders supporters tended to think her decision to support Clinton was uninformed, and often tried to mansplain things to her about the campaign that she already knew. "There's a lot of, 'If you knew this, you wouldn't vote for Hillary,'" she said. "Thinking that might sway a vote, if you just knew this one fact about Hillary."

Another perspective, from Iowa State University senior Monica Diaz, who has volunteered for Clinton:

Like other young Clinton supporters interviewed for this article, Diaz says Sanders backers will "talk at me for hours" without pausing to hear her reasoning. "Bernie supporters judge people who are not Bernie supporters, and they judge Hillary supporters the hardest," Diaz said in an email. "They think a vote for Hillary is a vote for Hitler."

(For more accounts on how real-life Bernie Bros are shouting over women, see here and here.)

The Sanders campaign, for its part, is aware of the problem and has asked fanatic supports to stand down. Mike Casca, rapid response director for Sanders, asked Sanders supporters to stand down:

This content is imported from Twitter. You may be able to find the same content in another format, or you may be able to find more information, at their web site.

if you support @berniesanders, please follow the senator's lead and be respectful when people disagree with you. — mike casca (@cascamike) January 26, 2016

Hector Sigala, digital media director for the Sanders campaign, told Mashable, "Sen. Sanders is committed to running a positive campaign on the issues, and we encourage all of his supporters to stick to that same message."

Sanders himself has addressed the issue. "People should not be attacking people," he said in an interview with Ebony Magazine, apologizing for some of his more zealot-like fans. "We have many hundreds of thousands of supporters, and some of them have gone over the edge."

Follow Prachi on Twitter.

Prachi Gupta Prachi Gupta is an award-winning journalist based in New York.

This content is created and maintained by a third party, and imported onto this page to help users provide their email addresses. You may be able to find more information about this and similar content at piano.io