Jeffrey Medford, a small-business owner in South Carolina, voted reluctantly for Donald Trump. As a conservative, he felt the need to choose the Republican. But some things are making him feel uncomfortable — parts of Mr. Trump’s travel ban, for example, and the recurring theme of his apparent affinity for Russia.

Mr. Medford should be a natural ally for liberals trying to convince the country that Mr. Trump was a bad choice. But it is not working out that way. Every time Mr. Medford dips into the political debate — either with strangers on Facebook or friends in New York and Los Angeles — he comes away feeling battered by contempt and an attitude of moral superiority.

“We’re backed into a corner,” said Mr. Medford, 46, whose business teaches people to be filmmakers. “There are at least some things about Trump I find to be defensible. But they are saying: ‘Agree with us 100 percent or you are morally bankrupt. You’re an idiot if you support any part of Trump.’ ”

He added: “I didn’t choose a side. They put me on one.”

Liberals may feel energized by a surge in political activism, and a unified stance against a president they see as irresponsible and even dangerous. But that momentum is provoking an equal and opposite reaction on the right. In recent interviews, conservative voters said they felt assaulted by what they said was a kind of moral Bolshevism — the belief that the liberal vision for the country was the only right one. Disagreeing meant being publicly shamed.