How Stephen Miller Rode an Anti-Immigration Wave to the White House

Once a lonely cause, restrictionism had grown into a mature movement — an intellectual ecosystem of sorts — with groups specializing in areas as diverse as litigation and voter mobilization.

When Mr. Sessions claimed on a conference call that the Gang of Eight bill threatened jobs, an analyst from the Center for Immigration Studies was on the line to vouch for the data, and Breitbart covered it as news. When the center presented its journalism award, Mr. Miller was the speaker, and his first-name references to the Center’s staff — “all the great work that Mark and Jessica and Steve are doing”— made it clear that he felt among friends.

Despite Mr. Sessions’s opposition, the bill passed in the Democratic Senate in 2013. As it headed to the Republican House, Mr. Miller drafted a 30-page memo that Mr. Sessions shared with the House Republican caucus, urging members to oppose the bill on behalf of “millions of struggling American workers.”

House leaders were mulling how to proceed when, in June 2014, an obscure Virginia professor toppled the majority leader, Eric Cantor, in a Republican primary. Though vastly outspent, the newcomer, Dave Brat, prevailed in large part by attacking Mr. Cantor for being “in cahoots” with Democrats on immigration.

“The world just changed,” Mr. Miller exulted the next day.

Indeed, it had. Among those commenting in Breitbart was the “conservative provocateur” Donald J. Trump, who said the upset showed that the Republican establishment was at risk. “Everybody is now vulnerable,” he said.

Circulating the article, Mr. Miller told friends that he wished Mr. Trump would run for president. When Mr. Trump did — demanding a wall and a ban on Muslims entering the country — Mr. Miller soon signed on.

Mr. Miller rose quickly on the small staff. A prolific writer and combative surrogate, he was the person most knowledgeable about the campaign’s central issue, and he lavished Mr. Trump with praise. (The Trump candidacy, Mr. Miller said, had altered “Western civilization.”) He also served as an ideological chaperone to a candidate given to sudden reversals of signature policies, a role Mr. Miller continues to play in the White House.