Some progressive activists who declined to back Mr. 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 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 A.L.S. — whom some Sanders-cheering accounts accused of lacking decision-making faculties due to his illness as he prepared to endorse Senator Elizabeth Warren — and Fred Guttenberg, the father of a shooting victim from the 2018 Parkland massacre, who had criticized Mr. Sanders’s statements about gun violence.

“Politics is a contact sport,” said Bakari Sellers, a former South Carolina State legislator who supported Ms. 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 Mr. Sellers, who is black, an “Uncle Tom” and wishing him brain cancer.

When Mr. Sanders’s 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.

For some perceived Sanders critics, there has been mail sent to home addresses — or the home addresses of relatives. The contents were unremarkable: news articles about the political perils of centrism. The message seemed clear: We know where you live.