Trump supporters are quick to turn on protesters, especially those who don’t look like them. They point and holler. Sometimes they spit and kick and shove. A young black woman in Kentucky was pushed and called names, her sign ripped from her hands. A black man in North Carolina was sucker-punched by a 78-year-old white man, who later looked into a camera and warned that next time, “We might have to kill him.”

To witness the crowd turn on the protesters in its midst is to watch a feverish body, bucking and writhing as it tries to eject an invading virus.

I have talked to protesters who still don’t quite have the words to describe what they felt when they were singled out and turned upon, often by their hometown communities. Mr. Trump says he condemns violence. But he also shouts at his crowds to “Get ’em out!” And even when he urges them not to hurt the protesters, a hard edge of menace bullets his words.

Yet the protesters, too, have sometimes instigated the clashes. They fling themselves to the ground, forcing local law enforcement officers — often outmanned and overwhelmed — to drag them away. They also shout and curse, making obscene gestures as they are led from events. And Friday night in Chicago, in perhaps the best-organized effort so far, they came not to simply stand quietly but to utterly halt Mr. Trump’s ability to deliver his speech.