Sometimes the self-declared greatest counterpuncher in the world lands a blow on the wrong person.

That was the case on Thursday night in Manchester, N.H., during a campaign rally when President Trump publicly ridiculed a man because of his weight; the president thought he was a protester. Instead, the man turned out to be a supporter of the president’s who was trying to help remove the actual protesters.

The episode took place midway through the rally, as Mr. Trump’s 90-minute speech was interrupted and the president paused to wait for the disruption to end. He looked up to the seats where it was taking place, and to a man wearing a Trump 2020 campaign shirt, and said, “That guy’s got a serious weight problem! Go home. Start exercising.” Mr. Trump then turned back to his speech and said his campaign was about “love.”

The president has come to be known for elementary insults about people’s appearances. But it is rare for him to go after supporters, whom Mr. Trump regularly flatters and praises in public.

The man with the “serious weight problem,” Frank Dawson, was standing in the arena near some protesters who were being ushered out. Mr. Dawson, donning his Trump gear, was fist-pumping the air as they were guided away.