Last year, in March, a group of Bernie Sanders voters spent days phone-banking staffers’ cell phones, imploring the campaign to challenge the vote in four states because of speculation on social media about ballot irregularities.

In April, they pointed to tweets from actor Tim Robbins alleging widespread voter fraud.

By the end of the primary, staunch Sanders supporters at state conventions would sit through the proceedings with their phones raised, videotaping. “There was a film-everything, everything’s-rigged mentality,” said Matt Berg, a former Sanders aide.

Two weeks into Donald Trump’s presidency, the same flashes of alarm, paranoia, and genuine outrage from the Democratic Party’s most fervent progressives have helped give rise to the left’s own distinct brand of the online phenomenon known as fake news.