This is a genuine topic of debate on the left. The consensus seems to be, “It’s OK, as long you call them a Nazi first.” It also helps if the punch is “funny.”
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.
Dollar-Menu magazine Newsweek thought seeing someone punched for their political beliefs was pretty good, but it became more “entertaining” when set to music. One of Obama’s speechwriters agreed.
Follow up question: Is it OK to shoot a leftist who assaults you because he defines your beliefs as “Nazi-ish.”