When Kashiya Nwanguma learned that Donald J. Trump would campaign in Louisville, Ky., where she is a student, she walked into a FedEx store and printed two colorful signs she had found online, depicting his head on a pig’s body.

Then she steeled herself for what has become the most provocative and potentially dangerous recurring act committed by ordinary voters in the 2016 presidential cycle: protesting Mr. Trump inside one of his own rallies.

The moment that Ms. Nwanguma, 21, who is black, held up her signs, Trump supporters ripped them away and began shoving her, screaming racial slurs and calling her “leftist scum,” she said in an interview.

“Did I enjoy being treated like trash? No, not at all,” she said.

At least she came away unharmed. The same could not be said for Rakeem Jones, 26, a protester who was punched in the face by a Trump supporter on Wednesday as law enforcement officers were leading him out of a campaign rally in Fayetteville, N.C.