We often consider something an ad only if it has money behind it. But sometimes a campaign-generated video can carry the weight of a political ad with no money behind it at all.

Michael R. Bloomberg has already spent more money on a presidential campaign than anyone else in history, plowing $338.7 million into traditional media, according to Advertising Analytics, a tracking firm. (Former President Barack Obama spent $338.3 million in the 2012 campaign, the firm said.) And yet, this week his team found particular success with a video it posted for free online.

The video, which catalogs vitriolic messages sent by some of Bernie Sanders’s online supporters, has been viewed more than four million times on Twitter and generated a news cycle’s worth of earned media coverage about the growing spat between the two candidates.

The message: The Democratic candidates for president are rarely aggressive in their social media attacks on one another. Their supporters, however, can be a different story, and a subset of Mr. Sanders’s online supporters has become notoriously known as “Bernie Bros.”

Mr. Bloomberg’s campaign decided to attack them head-on in its first wholly negative assault on a Democratic rival for president. It ran a rapidly accelerating highlight reel of some of the threats and abusive comments doled out by a selection of Sanders supporters, and framed the Vermont senator as responsible for their actions. After a clip showing Mr. Sanders calling for “civil discourse” at Liberty University in 2015, the ad goes black, with just text on the screen, flipping from “Really?” to “Really.”

(Mr. Sanders has said that “anybody making personal attacks against anybody else in my name is not part of our movement.”)

The takeaway: Though the online trolls supporting Mr. Sanders have been making waves for years, Mr. Bloomberg’s video was posted to Twitter in the wake of reports that leaders of a powerful Nevada labor union were inundated with online abuse and harassment after they chose not to endorse a candidate in the caucuses there.