Video of the attack quickly inspired a flood of jokes and memes online, some of which set the punch to songs like “Born in the U.S.A.” Jon Favreau, a former speechwriter for Barack Obama, was one of many who posted a comment.

There was little substantive debate online about the ethics of punching Mr. Spencer. Twitter is not a place where minds are often changed, and the supporters and opponents of the sucker punch were unmoved by one another’s quips.

Opponents of the punch tended to say that violence had no place in political debate. Supporters tended to say the punch was funny, and more than a few compared Mr. Spencer’s attacker to famous Nazi punchers from pop culture, like Indiana Jones and Captain America.