WASHINGTON — Peter Brimelow, the founder of the anti-immigration website VDARE, believes that diversity has weakened the United States, and that the increase in Spanish speakers is a “ferocious attack on the living standards of the American working class.”

Jared Taylor, the editor of the white nationalist magazine American Renaissance, is a self-described “white advocate” who has written that “newcomers are not the needy; they are the greedy.”

Their websites were among the sources cited by Stephen Miller, the White House aide who is the driving force behind President Trump’s immigration policies, in emails and conversations with conservative allies at Breitbart News when he was a young Senate aide. A cache of those emails, obtained by the Southern Poverty Law Center, provides new insight into the ideas that have shaped Mr. Miller’s thinking and suggest he has maintained deeper intellectual ties to the world of white nationalism than previously known.

“The heart of where these guys differ from neoconservatives and Republican orthodoxy is basically: ‘What is the American nation and what is the nature of American nationhood?’” Lawrence Rosenthal, the chair and lead researcher at the Berkeley Center for Right-Wing Studies at the University of California, said in an interview.