“I think what we really find troubling is the mainstreaming of these really offensive ideas,” said Jonathan Greenblatt, the national director of the Anti-Defamation League, which tracks hate groups. “It’s allowed some of the worst ideas into the public conversation in ways we haven’t seen anything like in recent memory.”

Mr. Trump declined to be interviewed for this article, and his spokesman declined to comment.

Outside a former aircraft factory in Bethpage, N.Y., not far from a strip of halal butchers and Indian restaurants now known as Little India, a Long Island housewife who gave her name as Kathy Reb finished a cigarette on a spring evening. Nervously, she explained how she had watched the complexion of her suburb outside New York City change. “Everyone’s sticking together in their groups,” she said, “so white people have to, too.”

The resentment among whites feels both old and distinctly of this moment. It is shaped by the reality of demographic change, by a decade and a half of war in the Middle East, and by unease with the newly confident and confrontational activism of young blacks furious over police violence. It is mingled with patriotism, pride, fear and a sense that an America without them at its center is not really America anymore.

In the months since Mr. Trump began his campaign, the percentage of Americans who say race relations are worsening has increased, reaching nearly half in an April poll by CBS News. The sharpest rise was among Republicans: Sixty percent said race relations were getting worse.

And Mr. Trump’s rise is shifting the country’s racial discourse just as the millennial generation comes fully of age, more and more distant from the horrors of the Holocaust, or the government-sanctioned racism of Jim Crow.

Some are elated by the turn. In making the explicit assertion of white identity and grievance more widespread, Mr. Trump has galvanized the otherwise marginal world of avowed white nationalists and self-described “race realists.” They hail him as a fellow traveler who has driven millions of white Americans toward an intuitive embrace of their ideals: that race should matter as much to white people as it does to everyone else. He has freed Americans, those activists say, to say what they really believe.

“The discussion that white Americans never want to have is this question of identity — who are we?” said Richard Spencer, 38, a writer and an activist whose Montana-based nonprofit is dedicated to “the heritage, identity and future of people of European descent” in the United States. “He is bringing identity politics for white people into the public sphere in a way no one has.”