WASHINGTON—After a brutal election that gutted Republican power in Washington, President Trump has chosen one of his most loyal and divisive advisers as the new White House chief of staff, signaling a combative posture for the final two years of the Trump era.

Stephen Miller, a fierce political operative who has been at Trump’s side for more than a decade, has been a vocal advocate of the no-holds-barred style of governing that delighted the president’s most die-hard MAGA supporters—and was repudiated by much of the rest of the country on Election Day. He succeeds Susie Wiles, who was seen as a moderating force on a sometimes impulsive president.

Miller, 41 years old, was the architect of the aggressive immigration enforcement policies that turned off many voters, according to polls, contributing to Republicans’ historic losses on Tuesday. His elevation signals that the president has no intention of backing away from the immigration agenda on which he built his political brand. Miller is expected to push a maximalist approach to executive power now that Democrats control Congress. In the days after the election, Trump sent early signals that he would work with Democrats, but Miller has long advocated for a confrontational response to the wave of investigations that the newly ascendant opposing party is expected to launch.

Though Miller has at times pushed for a muscular foreign policy, particularly in South America, he has suggested they wind down the deeply unpopular war in Iran, according to people familiar with the discussions, and called for refocusing on policies like immigration that made Trump popular with his base.

Miller, when contacted Friday, directed questions to the White House. “He’s going to do a great job,” Trump told reporters after arriving at an event in Florida. An early test for Miller, according to associates of the president, is whether he can satisfy Trump’s desire to punish his perceived political enemies.

One of the president’s biggest frustrations with Wiles, according to administration officials, is that she and the Justice Department weren’t doing enough to go after people seen as leaking classified material, to jail reporters who publish such material and to prosecute his opponents.

Though Miller was influential during Trump’s first term, his purview has greatly expanded in the second one. He has overseen policy rollouts concerning the president’s trade war and targeting of elite universities, along with spearheading the Pentagon’s unprecedented campaign of striking alleged drug-carrying boats in the Caribbean, killing many on board.

He hosts a daily 10 a.m. phone call with officials at agencies across the government to solicit new ideas to further the administration’s immigration crackdown. This week, Miller continued to lead those calls as usual, never acknowledging the election results on Tuesday or a need to change course, according to people familiar with the matter. His demeanor has alarmed some officials at U.S. Immigration and Customs Enforcement, who fear the country’s shift toward Democrats means their jobs will only grow more dangerous, the people said.

He has personally drafted or edited nearly every executive order the president has signed, and he has faced little opposition from administration officials to his work to reshape immigration policy.

Even some posts at cabinet agencies have been described by administration officials as reporting directly to Miller, effectively bypassing cabinet secretaries.

In recent weeks, he has worked closely with White House budget chief Russ Vought to chart a second-term agenda with an emphasis on executive actions that don’t require congressional approval, administration officials said.

Miller has also played a central role in planning for the White House response to congressional investigations, the officials said. Some in Washington, even people supportive of Trump, saw the president’s decision to promote Miller as a mistake that misreads the mood of the country. Rep. Marlin Stutzman (R., Ind.) called it a “horrible idea.”Miller has told associates he plans to try to limit the influence of lobbyists on the administration. Outmaneuvering othersMiller is known to keep long hours in the White House, colleagues say, and often calls deep into government agencies to issue edicts.

Several staffers said he knew far more about the details of certain agencies than Wiles, and often used that to outmaneuver others. He is deeply involved in shaping his public image, working closely with the White House press office, people familiar with the dynamics say. He has also become a central focus of socializing in Trump’s orbit. His wife, Katie Miller, a former Trump White House staffer, regularly throws parties and dinners at their residence on a military base, attracting cabinet and White House officials.

They live near Defense Secretary Pete Hegseth, and they socialize with him and his wife, Jennifer Hegseth, a former Fox News producer, according to people familiar with the matter. He has forged tight relationships with leading administration figures, including Secretary of State Marco Rubio. Less is known about his relations with Vice President JD Vance.

Miller has also asserted his influence over the FBI, even making at least one unusual appearance at the bureau’s headquarters to stress to agents the importance of the Trump administration’s priority of cracking down on threats of political violence from the left, people familiar with the meeting said.

He told agents nothing should stand in their way of investigating threats to public officials, referencing his own personal experiences. The president, aware of polls showing that much of his immigration agenda isn’t popular, has told advisers he wasn’t comfortable with how far Miller has gone on some fronts, according to people who have spoken with Trump. Trump has also heard complaints from business leaders about workers being thrown out of the country. One high-profile policy Miller spearheaded, forcing most green-card hopefuls to leave the country to apply for permanent residence, was quickly reversed by the White House after intense pushback from Silicon Valley executives.

Despite Miller’s often zealous views, the president has come to value him for his unwavering loyalty above all else. Miller has stood by the president even at his lowest moments, including after the Jan. 6, 2021, riot at the U.S. Capitol. Miller opposed both of Trump’s picks for Homeland Security secretary: Kristi Noem, and then Markwayne Mullin. But once Trump made up his mind, Miller offered full-throated support for each, both in public and in private, according to people familiar with the matter.

Last year, Miller shut down discussion about whether the U.S. should bomb Houthi targets in a Signal chat that was accidentally shared with a magazine editor. As the vice president and top national security officials discussed options, Miller weighed in. “As I heard it, the president was clear,” Miller said. “Green light,” he added.

Miller has been at the center of contentious issues between the White House and the nation’s biggest banks, including pushing for the government to take a hard-line stance in the Trump administration’s probes into allegations the firms improperly shut off accounts for political and religious reasons, according to people familiar with the matter.

Numerous people had been floated to succeed Wiles, including political allies Bill Pulte, Sergio Gor and Jay Clayton, people familiar with the matter said. But Miller won out on the argument that he has been through countless battles with Trump and is the kind of consigliere needed in the final two years of the administration. Miller also stood out as a believer in Trump’s trade agenda, unlike some other presidential advisers who saw tariffs as helping fuel cost-of-living concerns that dominated the election.

Katie Miller also encouraged people to promote him for the job, people familiar with the matter said. A person familiar with the matter said Miller also wanted to see a focus on the Make America Healthy Again agenda championed by Health Secretary Robert F. Kennedy Jr. The person added that unlike Wiles, who had grown frustrated with Hegseth, he would be more supportive of Hegseth.

Wiles’s downfallWiles, who was Trump’s 2024 co-campaign manager, lasted longer than any of Trump’s first-term chiefs of staff—he went through four in four years. She was an instrumental force in his return to power following his 2020 loss to President Joe Biden. And she surrounded herself with close allies from Florida, including James Blair, a young political operative who gained Trump’s admiration—only to draw the president’s wrath following this week’s midterms drubbing.

Trump has complained about Blair in recent weeks, and he is not expected to have a formal role in Trump’s orbit going forward, people familiar with the matter said. Wiles and other advisers had told Trump up until last weekend that the GOP could keep control of the Senate, and some faulted her for not setting better expectations. The woman Trump nicknamed the “Ice Maiden” had come under attack from the party’s most conservative forces, who saw her as holding Trump back. Conservative firebrand Steve Bannon publicly called on Friday for Wiles to be removed, not long before Trump made the news public on his Truth Social platform.

Podcaster Joe Rogan on Friday published an interview with Calley Means, a top ally to Robert F. Kennedy Jr., who left his position in the Trump administration last month. Means was critical of Wiles, saying she bent the White House to corporate interests and had a “stranglehold on any policy decision.”A White House official said Wiles had been an essential leader in making America healthy again.

Tensions erupted earlier this week. Trump yelled at Wiles in front of others on election night, according to people familiar with the matter. Even so, as late as Wednesday evening, Wiles’s supporters were saying she wouldn’t be ousted. By Thursday night, however, her allies were calling members of Congress, asking them to reach out to Trump and make supportive comments about her, according to a person with knowledge of the calls.

Even Trump has sometimes seen Miller as overly aggressive. He has been hawkish on violent crime, at times personally leading meetings with local police officials in Washington.“I would love to have him come up and explain his true feelings,” Trump said of Miller at an October 2025 Oval Office press conference about cracking down on crime. “Maybe not his truest feelings—that might be going a little bit too far.”The room broke out in laughter.

Write to Michelle Hackman at michelle.hackman@wsj.com, Josh Dawsey at Joshua. Dawsey@WSJ.com, Alex Leary at alex.leary@wsj.com and Sadie Gurman at sadie.gurman@wsj.com