It’s unlikely you have heard of @ToTheXToTheY. But you may be familiar with its work.

This anonymously run Twitter account made a splash in 2016, after a protester rushed Donald J. Trump at a rally in Dayton, Ohio.

The Secret Service nabbed the man before he got too close, but not before Mr. Trump flinched. That was an understandable reaction, but it created a potential image problem for the candidate, since he was the No. 1 tough guy in the Republican primary race at the time.

Soon after the incident, @ToTheXToTheY posted a doctored video that falsely linked the protester, a college student named Tommy DiMassimo, to ISIS. The bogus message from an account with some 5,000 followers would have gone all but unnoticed, but then Mr. Trump retweeted it.

The candidate also added a bit of commentary: “USSS did an excellent job stopping the maniac running to the stage. He has ties to ISIS. Should be in Jail!” And he stood by the hoax video a few days later on “Meet The Press.”