The defense from Bernie Sanders was straightforward: It wasn't me.

He had been milling about on the Senate floor one day in the summer of 2017 when a colleague, Kamala Harris, stepped toward him. "Do we have a problem?" Harris asked, according to Democrats familiar with the exchange.

Some prominent Sanders supporters had been flaming Harris publicly as the preferred choice of the corporate Democratic establishment against which Sanders had long railed, a view amplified among Sanders-boosting accounts across social media. "Pre-emptive strike," one person wrote on the popular SandersForPresident Reddit group, where Sanders fans were sharing details of Harris' recent fundraising swing in the Hamptons with former Hillary Clinton donors. "Start the conversation now, end it before 2020."

Sanders assured Harris that there was no issue, the Democrats familiar with their conversation said. He insisted that he could not control how his followers communicated.

But two years later, as both senators pursued the party's 2020 presidential nomination and Harris returned to the Hamptons to collect campaign checks, Sanders broadcast an observation of his own after Harris raised doubts about his "Medicare for All" plan. "I don't go to the Hamptons to raise money from billionaires," he tweeted last August, elevating a message that supporters had already been pushing. Thousands of retweets followed.

Since the start of Sanders' first presidential campaign in 2016, his colossal online support base has been by turns a source of peerless strength and perpetual aggravation — envied and caricatured by rivals who covet such loyalty, feared by Democrats who have faced harassment from his followers, and alternately cherished and gently scolded by the candidate himself.

The zeal of Sanders' fans has helped establish him as one of the 2020 front-runners a week before the Iowa caucuses. No other Democrat attracts supporters more dedicated to forcefully defending their candidate and lashing his foes, more willing to repeatedly donate their time and money to sustain his bid. Through the end of 2019, Sanders had raised nearly $100 million from more than 5 million individual donations, without ever holding traditional fundraisers, leading the primary field.

Yet as Sanders moves to position himself as a standard-bearer for a party he has criticized from the left for decades, the power of his internet army has also alarmed Democrats who are familiar with its underside, experienced in ways large and small.

Some progressive activists who declined to back Sanders have begun traveling with private security after incurring online harassment. Several well-known feminist writers said they had received death threats. A state party chairwoman changed her phone number. A Portland, Ore., lawyer saw her business rating tumble on an online review site after tussling with Sanders supporters on Twitter.

Other notable targets have included Ady Barkan, a prominent liberal activist with ALS — whom some Sanders-cheering accounts accused of lacking decision-making faculties due to his illness as he prepared to endorse Sen. Elizabeth Warren — and Fred Guttenberg, the father of a shooting victim from the 2018 Parkland school massacre in Florida, who had criticized Sanders' statements about gun violence.

"Politics is a contact sport," said Bakari Sellers, a former South Carolina state legislator who supported Harris in the Democratic primary. "But you have to be very cognizant when you say anything critical of Bernie online. You might have to put your phone down. There's going to be a blowback, and it could be sexist, racist and vile."

In recent days, he said, one man sent a profanity-filled private message on Instagram, calling Sellers, who is black, an "Uncle Tom" and wishing him brain cancer.

When Sanders' supporters swarm someone online, they often find multiple access points to that person's life, compiling what can amount to investigative dossiers. They will attack all public social media accounts, posting personal insults that might flow in by the hundreds. Some of the missives are direct threats of violence, which can be reported to Twitter or Facebook and taken down.

More commonly, there is a barrage of jabs and threats sometimes framed as jokes. If the target is a woman, and it often is, these insults can veer toward her physical appearance.

The Sanders campaign declined to discuss its 2020 digital operation and the extent to which it monitored social media discussions.

A spokesman, Mike Casca, flagged Sanders' call for civility from last February. The campaign also released a statement from a spokeswoman, Sarah Ford, emphasizing the candidate's previous remarks. "As the senator has said loudly and clearly," she said, "there is no room in the political revolution for abuse and harassment online."

But many political veterans outside the Sanders operation fault the campaign's handling of the vitriol.

Jess Morales Rocketto, a progressive strategist who worked on campaigns for Barack Obama and Hillary Clinton, said Sanders had empowered aides and surrogates who "have a tendency to aggressively amplify things that a campaign would normally shut down amongst supporters."

"There are always people who say things that are problematic. It's not that that is unique to Bernie's campaign," she said. "What's unique is it is a consistent problem in the universe of Bernie Sanders."

With more than 10 million followers on Twitter, Sanders has a larger audience on the platform than Warren, Pete Buttigieg, Joe Biden and Sen. Amy Klobuchar combined. A sizable number could be automated bots or fictitious accounts. Federal prosecutors have detailed coordinated efforts by Russian nationals to interfere in the 2016 election, with an emphasis on two candidates — Donald Trump and Sanders — whom the Russians hoped to bolster while denigrating their opponents.

In a party gripped with anxiety about unifying to defeat Trump, the venom among Sanders backers and their counterparts supporting other candidates is of serious concern to Democrats.

Peggy Huppert, an Iowa activist who consulted for the 2016 Sanders campaign, said she had decided to support Buttigieg, the former mayor of South Bend, Ind., in 2020 "in large part because of the way he conducts himself." She praised Sanders' letter to supporters after his announcement but said that his message had plainly failed to resonate.

"Obama set the tone for his campaign: 'You are positive, you are respectful, you are civil,' " Huppert said. "I guess Bernie hasn't."