This has come up repeatedly during Mr. Trump’s presidency, whether it be the white supremacists who marched in Charlottesville, Va., or the bomber who sent explosives to Mr. Trump’s political adversaries and prominent news media figures or the gunman who stormed a Pittsburgh synagogue after ranting online about “invaders” to the United States.

David Livingstone Smith, a philosophy professor at the University of New England and the author of a book on dehumanization of whole categories of people, said Mr. Trump had emboldened Americans whose views were seen as unacceptable in everyday society not long ago.

“This has always been part of American life,” he said. “But Trump has given people permission to say what they think. And that’s crack cocaine. That’s powerful. When someone allows you to be authentic, that’s a very, very potent thing. People have come out of the shadows.”