Twitter isn’t real life. But for Sanders fans, it’s a powerful tool.

“The Democratic party is a progressive party,” Elizabeth Warren’s official account tweeted last week, “even if there are a lot of people on the #DemDebate stage who don’t want to say so.” It was a jab at all the other Democratic candidates except for Bernie Sanders. And it’s easy to imagine Senator Sanders’ supporters “liking” that message.

But that’s not what happened.

The first response came one minute later from an account called @AmazingBernie, and it featured a photoshopped image of Senator Warren’s head on the body of a snake. Soon dozens of replies from Sanders supporters piled on top. “You’re a pathological liar,” wrote someone named Paul. “Go away Republican,” tweeted Robin. “YOU ARE NOT A PROGRESSIVE!” thundered an account named “I’m 1 of the Squad.” 

Editor’s Note: Social media accounts operated by one of the people quoted in this story, who said his name was Alan Jeffs, have subsequently been shut down by Twitter and Facebook, after a Washington Post investigation raised questions about Mr. Jeffs’s identity. The account cited at the time had been verified as authentic. Mr. Jeffs’s phone has since been disconnected.

It’s a recurring pattern. A candidate from a rival campaign sends out a tweet, and within minutes it is swarmed by hundreds, or thousands, of responses that are either supportive of Mr. Sanders, scathing towards his rivals, or both. A Monitor analysis of several hundred Twitter accounts – those most frequently responding directly to the five leading Democratic candidates over the past year – using data provided by researchers at George Washington University, found that the number of pro-Sanders accounts far exceeds supporters of any other Democratic campaign.  

And while it’s become almost cliché to note that Twitter isn’t “real life,” that doesn’t mean it has no impact. During the 2016 campaign, Donald Trump proved Twitter’s power in agenda-setting, and he has continued to use it as an effective tool throughout his presidency. Among the thousands of campaign-related tweets analyzed by the Monitor, only Mr. Trump’s supporters rival Mr. Sanders’ in the quantity and often vitriolic nature of their replies.

Indeed, in the Democratic primary race, Sanders supporters have dominated the Twitterverse in a way that data experts and political scientists say is unlike anything they’ve ever seen. 

“This may be a permanent change in politics,” says Frank Sesno, director of George Washington University’s School of Media and Public Affairs. “Any candidate, when they are looking at what it takes to succeed, will come away seeing that now they have to appeal even more to the emotions, and the fears, of their followers – because that emotion drives response and engagement on social media.”

With Mr. Sanders racking up wins in two of the first four primaries, the behavior of his supporters has come under sharper scrutiny. Recently, the Bloomberg campaign launched an ad on Twitter featuring screenshots of online threats made by Sanders fans. At the Nevada debate, Pete Buttigieg asked Mr. Sanders directly why such behavior seems to be so prominent among his supporters.

“We do not want your support if you think that what our campaign is about is making ugly attacks on other candidates,” Mr. Sanders said last week, when pressed on the issue during a CNN town hall. “We don’t want you. You’re not part of us.”

The Vermont senator has also suggested that some of the online vitriol may actually be Russian efforts to once again interfere in a United States election. This claim was bolstered by news that Russia may be trying to help Senator Sanders win the Democratic nomination. 

But Twitter has denied this claim, saying it would identify and disclose such activity. And according to several social media experts, the most frequent pro-Sanders tweeters are likely real people. Since the 2016 election, Twitter has made it much more difficult and expensive to create fake accounts.

“I don’t think we can blame this on Russia,” says Trevor Davis, a research professor at George Washington University’s Institute for Data, Democracy and Politics. “A subset of Sanders supporters shares a deep distrust in the party,” he says. “Their attacks on other Democrats reflect a belief that the system is rigged.”

Of the 100 accounts that have most frequently responded to Mr. Buttigieg’s tweets, before he dropped out of the race over the weekend, at least 55 are identifiably pro-Sanders. Among Mr. Bloomberg’s top repliers, 51 are pro-Bernie.

And of Ms. Warren’s most frequent repliers, 25 are pro-Bernie – fewer than the others, but in some ways the most telling, since they are undermining the candidate who is ideologically closest to their own. None of Mr. Sanders’ top 100 repliers, on the other hand, are from identifiably pro-Warren accounts.  

“Our goal is to influence the discourse”

Britin Foster, a musical booking agent who lives near Albany, New York, tweets and retweets hundreds of pro-Bernie messages a day, in addition to making hundreds of replies to other candidates’ tweets. 

When informed that she’s one of the top 100 repliers to both Mr. Bloomberg and Mr. Buttigieg, she says she’s honored by that distinction – particularly her frequent replies to Mr. Bloomberg, whom she regards as even worse than President Trump. One of the memes featured in the former mayor’s commercial about online threats made by Sanders supporters, says Ms. Foster proudly, was hers. 

She isn’t at all surprised that Sanders supporters’ Twitter activity dwarfs everyone else’s. That, after all, is the goal.

“Calling Bloomberg out, for example – we can do that on Twitter, and the media sees it, the pundits see it, the other politicians see it,” says Ms. Foster. “Our goal is to influence the national discourse ... and we don’t have another platform where we could reach those people so easily.” 

After MSNBC host Chris Matthews made comments on air comparing Sanders supporters to the Germans in World War II, Ms. Foster saw the outrage build on Twitter, with Sanders supporters producing memes and tweets that multiplied exponentially in no time. Last week, Mr. Matthews made a public apology for his comments.  

“If social media weren’t worthwhile, then why is Mike Bloomberg spending hundreds of millions to advertise on it?” says Alan Jeffs, a supporter of Mr. Sanders who has already made the list of Mr. Bloomberg’s top responders since starting his account @BernieOrElse a few weeks ago. “Twitter is the real world now, even more than it was four years ago.” 

Asked about the vulgarity and outright hostility of many Sanders supporters online, both Ms. Foster and Mr. Jeffs justify it as in the service of a greater good. If it helps Mr. Sanders get elected, they say, it will be worth it. Sanders supporters aren’t locking children in cages, says Mr. Jeffs; they’re trying to ensure everyone has access to health care. Mr. Jeffs himself is currently unemployed, and says he quit his job as a graphic designer just to qualify for Medicaid, because the Affordable Care Act didn’t cover his prescribed treatments.

“It may feel like it’s bullying or like it’s blackmail, but the lower and working classes have been bullied by the establishment and elite for too long,” says Mr. Jeffs, adding that Mr. Sanders’ supporters are just “better at the internet” than other candidates’ supporters. 

Indeed, many Sanders supporters believe the news media is biased against the Vermont senator, and that it’s up to them to hold his more moderate rivals to account.

“Sometimes it does come out vehemently, because we see our friends, our family, our communities suffering,” says Ms. Foster. “We are upset with people who are trying to maintain the status quo, when the status quo has been shown to be enormously harmful.”

A challenge for party unity

Being “better” at Twitter doesn’t necessarily give Sanders supporters a direct line to the American electorate. A 2019 Pew Research survey found that just 22% of U.S. adults use Twitter – fewer than one-third of those on Facebook. And within this segment of the population, it’s an even smaller segment making the loudest noise, with the top 10% of tweeters – who are much more likely to be young Democratic women –  generating 80% of all tweets. 

“I’m skeptical that replies on Twitter are going to affect anybody’s vote,” says Nicco Mele, a lecturer with Harvard University’s Kennedy School who worked as webmaster on Howard Dean’s groundbreaking 2004 digital campaign. Twitter helped Mr. Trump win in 2016 because of how he himself used it – and still uses it – to inform the news cycle, says Mr. Mele, not because of what his supporters are tweeting. 

“It’s this simple: I don’t see on CNN, ‘Candidate Bernie just tweeted this’ or ‘Bernie supporters just tweeted that,’” says Mr. Mele. “Every day, Trump is using Twitter to drive the storyline and drive the media, and Bernie supporters aren’t doing that.”

Still, others warn that the vast pro-Sanders efforts directed against his Democratic rivals online may cause harm to the party’s ability to unify, the full extent of which may not become clear until November. 

“If the Democrats want to regain the White House, they will have to find a way to unite behind the nominee,” says Mr. Davis. “There are some very loud voices online that could make that difficult.”