“Sleeping Giants’ political playbook is to attack opposing speech through harassment and false claims to try to drive it out of business,” Mr. Glicklich said. “They and others have failed at this every time it has been attempted. Democracy flourishes from more conversation, not less, which is why Matt Rivitz’s speech suppression through economic force is among the most reviled techniques of coercion.”

Shortly after Mr. Rivitz started the account, it caught the attention of Ms. Jammi, an American who lives in Berlin. She also visited Breitbart after the election and was startled to see ads from major companies there, thanks to her browsing history. Mr. Rivitz contacted Ms. Jammi through Twitter after seeing a post she had written for Medium on how marketers could blacklist Breitbart, and the two joined forces.

Ms. Jammi, who said her previous interest in politics had amounted to nothing more than following the news, discussed anonymity “early and often” with Mr. Rivitz.

“Initially, we were kind of freaked out at the alt-right influence, and, obviously, one of our primary concerns was staying safe,” she said.

Knowledge of their involvement was limited to a tight circle of family and friends. Mr. Rivitz and Ms. Jammi, who have met in person once, said they each spent three to eight hours a day on Sleeping Giants — posting tweets and corresponding with companies and advertisers — while working at their day jobs. They were vague about how many other people help run the account and its Facebook page, citing privacy concerns and threats.

Since Sleeping Giants got its start, a great number of brands have taken steps to make sure that they do not appear on Breitbart. The site had about 649 advertisers on its website last month, showing around 1,902 different display ads, according to data from Moat Pro, a digital ad intelligence product. That was down from 3,300 advertisers and 11,500 display ads in November 2016. (Sleeping Giants’ count of departed advertisers is closer to 4,000.)

As Sleeping Giants expanded, it broadened its mission to making “bigotry and sexism less profitable” over all. In April 2017, it rallied its following to join the widespread pressure on companies that advertised on “The O’Reilly Factor” after The New York Times reported that Bill O’Reilly, the Fox News show’s host, had settled with at least five women who had accused him of harassment. This year, the account added to the pressure on brands whose commercials appeared on “The Ingraham Angle,” the Fox News show hosted by Laura Ingraham, after she ridiculed a student survivor of the Parkland, Fla., school shooting.