One afternoon in November, a half-dozen government officials sat at a conference table in the White House, waiting for the arrival of Stephen Miller, a senior adviser to Donald Trump. Miller had summoned officials from the Departments of Homeland Security, State, and Justice to discuss a new Administration policy initiative: a series of agreements with the governments of Central America that would force asylum seekers to apply for protection in that region instead of in the United States. Miller, who had helped make the deals, wanted to know when their provisions could go into effect. Typically, everyone rises when top White House officials enter a room. But when Miller walked in, wearing a dark suit and an expression of wry resolve, everyone remained seated, their eyes cast down. “You go into meetings with Miller and try to get out with as little damage as possible,” a former Administration official told me. Miller has a habit of berating officials, especially lower-ranking ones, for an agency’s perceived failures. Chad Wolf, now the acting head of D.H.S., used to advise colleagues to placate Miller by picking one item from his long list of demands, and vowing to execute it. “It’s a war of attrition,” Wolf told them. “Maybe he forgets the rest for a while, and you buy yourself some time.”

One participant in the November meeting pointed out that El Salvador didn’t have a functioning asylum system. “They don’t need a system,” Miller interrupted. He began speaking over people, asking questions, then cutting off the answers.

As the meeting ended, Miller held up his hand to make a final comment. “I didn’t mean to come across as harsh,” he said. His voice dropped. “It’s just that this is all I care about. I don’t have a family. I don’t have anything else. This is my life.”

Miller, who is thirty-four, with thinning hair and a sharp, narrow face, is an anomaly in Washington: an adviser with total authority over a single issue that has come to define an entire Administration. “We have never had a President who ran, and won, on immigration,” Muzaffar Chishti, of the Migration Policy Institute, told me. “And he’s kept his promise on immigration.” Miller, who was a speechwriter during the campaign, is now Trump’s longest-serving senior aide. He is also an Internet meme, a public scourge, and a catch-all symbol of the racism and malice of the current government. In a cast of exceptionally polarizing officials, he has embraced the role of archvillain. Miller can be found shouting over interviewers on the weekend news shows or berating reporters in the White House briefing room; he has also vowed to quell a “deep state” conspiracy against Trump. When he’s not accusing journalists of harboring a “cosmopolitan bias” or denying that the Statue of Liberty symbolizes America’s identity as a nation of immigrants, he is shaping policy and provoking the President’s most combative impulses.

Jeh Johnson, who headed the Department of Homeland Security under Barack Obama, told me, “D.H.S. was born of bipartisan parents in Congress, in the aftermath of 9/11, when there was support for a large Cabinet-level department to consolidate control of all the different ways someone can enter this country.” D.H.S. is the third-largest federal department, with a fifty-billion-dollar budget and a staff of some two hundred thousand employees, spanning the Federal Emergency Management Agency, the Cybersecurity and Infrastructure Security Agency, and Immigration and Customs Enforcement. From its founding, in 2002, to the end of Obama’s Presidency, the department had five secretaries; under Trump, it has had five more. “Immigration is overheated and over-politicized, and it has overwhelmed D.H.S.,” Johnson said.

“The massive changes Miller engineered in border and immigration policy required that the policymaking process at D.H.S. be ignored,” Alan Bersin, a former senior department official, told me. “Who do you think has filled the vacuum?” Miller has cultivated lower-level officials in the department who answer directly to him, providing information, policy updates, and data, often behind the backs of their bosses. “At the beginning of 2017, none of us could have foreseen that he would wield this kind of power,” a former Trump Administration official told me. Of thirty current and former officials I interviewed, not one could recall a White House adviser as relentless as Miller, or as successful in imposing his will across agencies. These officials resented him as an upstart and mocked his affectations—his “arrogant monotonal voice” and tin-eared bombast—but few were comfortable going on the record, even after leaving the government. Miller is famously vindictive, and, as Trump runs for a second term, he is sure to grow only more powerful. “Miller doesn’t have to get Trump to believe everything he does,” one of the officials told me. “He just has to get Trump to say it all.”

When Miller and I spoke by phone, it was off the record. Without an audience, he gave the same message at half the volume—a litany of talking points about all the ways in which the President had delivered on his campaign promises. Afterward, the White House sent me a quote for attribution: “It is the single greatest honor of my life to work for President Trump and to support his incredible agenda.”

Miller’s obsession with restricting immigration and punishing immigrants has become the defining characteristic of the Trump White House, to the extent that campaigning and governing on the issue are no longer distinguishable. In the past three and a half years, the Trump Administration has dismantled immigration policies and precedents that took shape in the course of decades, using current laws to intensify enforcement against illegal immigration and pursuing new ones to reduce legal immigration. Trump has slashed the refugee program; virtually ended asylum at the southern border; and written a rule denying green cards to families who might receive public benefits. Miller has choreographed these initiatives, convincing Trump that his political future depends on them—and on going even further. If Trump is not reëlected, Miller will never again have such power. A D.H.S. official told me, “Going into 2020, Miller is at a crossroads.”

The radicalism of Miller’s views tends to obscure how much he has evolved as a tactician since he arrived in Washington. He grew up in Santa Monica, California, the son of Jewish Democrats, but, by the time he entered high school, he had become a strident conservative. “He was going to a very liberal, diverse school,” Megan Healey, one of his classmates, told me. “In a school where the nerds were considered cool, he was still the guy that nobody liked.” The terrorist attacks of 9/11 took place when he was a junior, cementing his persona. “Anti-Americanism had spread all over the school like a rash,” he later wrote. “Osama Bin Laden would feel very welcome at Santa Monica High School.” At Duke, where he studied political science and wrote a column for the student newspaper, he became a familiar presence on conservative television and radio programs. His hostility toward immigrants formed part of his politics, but did not stand out. He opposed left-wing bias in the classroom, invited controversial speakers to campus, and organized “Islamo-Fascism Awareness Week.” “America without her culture is like a body without a soul,” he wrote in one column. “Yet many of today’s youth see America as nothing but a meeting point for the cultures of other nations.” His most notable cause was to defend a group of white lacrosse players who had been falsely accused of raping a black woman who was stripping at a party. The editor of his column later told The Atlantic, “He picked the most contrarian of stances to articulate, wrote the most hyperbolic prose he could . . . then sat back and waited for people’s reactions.”