A Fourth of July Symbol of Unity That May No Longer Unite

SOUTHOLD, N.Y. — The American flag flies in paint on the side of Peter Treiber Jr.’s potato truck, a local landmark parked permanently on County Route 48, doing little more, he thought, than drawing attention to his family’s farm.

Until he tried to sell his produce.

At a local greenmarket where he sells things like wild bergamot, honey and sunflowers, he had trouble striking a deal until, he said, he let his liberal leanings slip out in conversation with a customer.

“She said, ‘Oh, whew. You know, I wasn’t so...