Jack Posobiec, a journalist with the far-right news outlet The Rebel, was the first to use the hashtag with a link to the hacked documents online, which was then shared more widely by WikiLeaks. Mr. Prosobiec remains the second-most mentioned individual on Twitter in connection with the hashtag behind WikiLeaks, according to a review of the past 100,000 Twitter messages posted since late Friday.

While there is no evidence that the recent hack against Mr. Macron’s campaign was organized by this loosely connected group of extremist campaigners, the American activists have been regularly gathering on sites like 4Chan and Discord, which was previously used to coordinate support for Donald J. Trump’s presidential campaign.

One popular tactic, according to experts, has been so-called Twitter raids, or efforts to hijack trending hashtag and topics on the social media site and inject far-right and anti-Macron propaganda.

A week before the second round of the French election, for instance, online activists, many from the United States and other English-speaking countries, flooded Twitter with coordinated anti-Macron memes — online satirical photos with often-biting captions — carrying hashtags like #elysee2017 that were linked to the campaign. That included portraying him as a 21st-century equivalent of Marie Antoinette, the out-of-touch last queen of France, and other memes linked him to false allegations of an extramarital affair.