On Sunday afternoon, former President Donald J. Trump dropped by a McDonald’s in Bucks County, Pa. He had cooked up a stunt to troll Vice President Kamala Harris, who has talked about having worked at McDonald’s one summer during college. Inside the restaurant, Mr. Trump wore an apron and dropped French fries into a vat of gurgling oil. Across the street, something much unhealthier was bubbling up.

A few hundred Trump supporters were lining the shoulder of the road and holding a tailgate party in the parking lot of a strip mall right where Philadelphia ends and the suburbs begin. Another group of locals — maybe 50 people — had turned up to protest Mr. Trump’s visit. People on the two sides spent the sunny autumn afternoon screaming into one another’s faces while filming the skirmishes on their iPhones.

The parking lot throbbed with hatred, fear and neighbor’s suspicion of neighbor. It became a microcosm of this year’s election, vicious and absurd. There was shouting about Project 2025 and the Jan. 6 riot. Transgender youth and vaccines. Tariffs and abortion. Fascism and communism. Mr. Trump’s supporters wore T-shirts that said “I’m voting for the convicted felon.” The other side yelled, “Lock him up.” One person wore an orange prison jumpsuit and a mask of Mr. Trump’s face.