One night in September 2014, when he was chief executive of Breitbart News, Stephen Bannon hosted cocktails and dinner at the Washington townhouse where he lived, a mansion near the Supreme Court that he liked to call the Breitbart Embassy. Beneath elaborate chandeliers and flanked by gold drapes and stately oil paintings, Jeff Sessions, then a senator from Alabama, sat next to the guest of honor: Nigel Farage, the insurgent British politician, who first met Sessions two years earlier when Bannon introduced them. Farage was building support for his right-wing party by complaining in the British press about “uncontrolled mass immigration.” Sessions, like other attendees, was celebrating the recent collapse in Congress of bipartisan immigration reform, which would have provided a path to citizenship for some undocumented people. At the dinner, Sessions told a writer for Vice, Reid Cherlin, that Bannon’s site was instrumental in defeating the measure. Sessions read Breitbart almost every day, he explained, because it was “putting out cutting-edge information.”

Bannon’s role in blocking the reform had gone beyond sympathetic coverage on his site. Over the previous year, he, Sessions and one of Sessions’s top aides, Stephen Miller, spent “an enormous amount of time” meeting in person, “developing plans and messaging and strategy,” as Miller later explained to Rosie Gray in The Atlantic. Breitbart writers also reportedly met with Sessions’s staff for a weekly happy hour at the Union Pub. For most Republicans in Washington, immigration was an issue they wished would go away, a persistent source of conflict between the party’s elites, who saw it as a straightforward economic good, and its middle-class voting base, who mistrusted the effects of immigration on employment. But for Bannon, Sessions and Miller, immigration was a galvanizing issue, lying at the center of their apparent vision for reshaping the United States by tethering it to its European and Christian origins. (None of them would comment for this article.) That September evening, as they celebrated the collapse of the reform effort — and the rise of Farage, whose own anti-immigration party in Britain represented the new brand of nativism — it felt like the beginning of something new. “I was privileged enough to be at it,” Miller said about the gathering last June, while a guest on Breitbart’s SiriusXM radio show. “It’s going to sound like a motivational speech, but it’s true. To all the voters out there: The only limits to what we can achieve is what we believe we can achieve.”

The answer to what they could achieve, of course, is now obvious: everything. Bannon and Miller are ensconced in the West Wing, as arguably the two most influential policy advisers to Donald J. Trump. And Jeff Sessions is now the attorney general of the United States. The genesis of their working relationship is crucial to understanding the far-reaching domestic goals of the Trump presidency and how the law may be used to attain them over the next four years. Bannon and Sessions have effectively presented the country’s changing demographics — the rising number of minority and foreign-born residents — as America’s chief internal threat. Sessions has long been an outlier in his party on this subject; in 2013, when his Republican colleagues were talking primarily about curbing illegal immigration, he offered a proposal to curb legal immigration. (It failed in committee, 17 to one.)

Talking to Bannon on air in September 2015, Sessions, who has received awards from virulently anti-immigrant groups, described the present day as a dangerous period of “radical change” for America, comparing it to the decades of the early 20th century, when waves of immigrants flooded the country. He said that the 1924 immigration quota system, which barred most Asians and tightly capped the entry of Italians, Jews, Africans and Middle Easterners, “was good for America.” Bannon is also uncomfortable with the changing face of the country. “When two-thirds or three-quarters of the C.E.O.s in Silicon Valley are from South Asia or from Asia, I think — ” he said on the radio with Trump in November 2015, vastly exaggerating the actual numbers. “A country is more than an economy. We’re a civic society.”