CHAPPAQUA, N.Y. — First came the grimaces — the disbelieving eyes trained upon his shirt, his signs, his car window stickers broadcasting a local betrayal.

Then his teenage son refused to drive the family car without redecorating, hiding the three “Make America Great Again” hats that usually sit atop the dashboard.

And there are deeper indignities still — more piercing than any parking lot glowering or dinner table slight — when John Nadler feels the full weight of his status as America’s loneliest supporter of Donald J. Trump: Acquaintances in the hometown Mr. Nadler has known for nearly three decades will not look him in the eye.

“People I’ve known for years,” Mr. Nadler, 63, said. “Just nod or wink or something. Anything.”

Across the country, every election introduces a measure of neighborly strain. Allegiances splinter. Eyes roll. Conversations curdle.