The Oval Office, which aides over the years have privately compared to Grand Central Terminal, will continue to be a bustling hub of activity under Miller’s watch, with advisers cycling in and out.One week into his tenure as chief of staff, Miller is charting a different path from his predecessors in the job, some of whom sought to put strictures on a president who has long chafed at efforts to manage him.President Trump is reflected in the sunglasses of Susie Wiles, the former chief of staff.Miller, people close to him say, plans to help steer the White House back to the MAGA movement’s ideological roots, reviving the hard-line, unapologetically populist agenda that defined Trump’s first rise to power and putting it at the center of the administration’s remaining years.While that approach is likely to appeal to the president’s most ardent supporters, it has been met with skepticism—and outrage—from Democrats and some Republicans, who worry that Miller’s elevation means that Trump hasn’t learned from the GOP’s midterms losses. Hakeem Jeffries, who is set to become the speaker of the House next year, has said he would refuse to work with Miller, adding that the new chief of staff “should be eradicated like a malignant tumor.” Retiring Republican Sen.

Thom Tillis called Miller “the worst of all possible choices.”White House spokeswoman Taylor Rogers said Miller had been “one of the most effective implementers of federal policy and bureaucratic reform in Washington, D.C.,” adding, “In his role as chief of staff, he will continue working to implement President Trump’s populist agenda on behalf of the nation’s working people.”Since starting his role Nov. 6, Miller has moved to reshape the administration’s policymaking process.

He has organized a marathon series of meetings with cabinet secretaries and senior advisers to the president. He began gearing up for a confrontation with Democrats on Capitol Hill, who are preparing to launch dozens of investigations into the administration.And he has already had to navigate a crisis, as an international espionage scandal widens, pitting two of Trump’s cabinet secretaries against each other.Stephen Miller meeting inside the West Wing.A longtime political adviser and one of the chief architects of Trump’s hard-line immigration policies, Miller, 41, has been by Trump’s side since he launched his bid for the White House more than a decade ago.

Now, as Trump enters the final stretch of his presidency, Miller will lead the West Wing through what is likely to be a consequential and combative final chapter of the administration.In a sign of Miller’s approach to the next two years, the desk in his spartan new office in the West Wing is adorned with a red hat with the word “VETO” plastered across it. This week, Miller ramped up the administration’s combative posture toward Democrats.

The White House warned the incoming chairman of the House Oversight Committee that some verbal threats from Democrats against executive branch officials “may cross the line into blatant criminality.”Miller is among a small group of Trump aides who are seen as fiercely loyal and protective of the president. That group includes Dan Scavino, Trump’s former golf caddie turned adviser.

Miller was the best man at Scavino’s Mar-a-Lago wedding this spring.“We need to stand with the man who has stood with us with every fiber of his being,” Miller told a crowd of conservatives at Trump’s Mar-a-Lago resort the day his promotion was announced. “I pledge you, Mr. President, my absolute loyalty, my unwavering commitment and my total devotion to your success and the success of this country.” Trump, who invited his new chief of staff to make the impromptu remarks, later shared a video of the speech to his millions of Truth Social followers.Stephen Miller has pledged absolute loyalty to Trump.A person close to Miller said that he has spent years studying the inner workings of White House operations.Over the past week, Miller has held one-on-ones with cabinet secretaries and agency and department heads.

In the meetings, he has taken stock of what is working and what changes should be made in the final two years of Trump’s administration, according to administration officials. He has handed over to a deputy the daily 10 a.m. interagency call on immigration that he attended religiously before he got promoted.A person close to Miller said the new chief of staff, who served in both Trump administrations, has spent years studying the inner workings of White House operations and the wider government.

He has long been known to get involved in the weeds of policy debates, reaching out directly to lower-level staff at agencies to encourage them to move more quickly.Miller began overhauling the West Wing with two memos that he issued to staff this week.WSJ’s Marianne LeVine explains President Trump’s move to appoint longtime adviser Stephen Miller his chief of staff. Aaron Schwartz/Press PoolIn the first memo, which was reviewed by The Wall Street Journal, he made Caleb Orr, one of Marco Rubio’s top advisers at the State Department, deputy chief of staff for policy.

He instituted new decentralized processes meant to empower individual cabinet secretaries. Separately, Miller invited his longtime ally May Mailman, a former deputy assistant to the president and senior policy strategist, to return to the White House on a temporary basis to help revamp operations across the administration.Miller has told associates he wants to expose Trump to as many people, ideas and disagreements as possible to help inform his decision-making.In a second memo sent a few days later, he forbade staff from meeting with lobbyists on White House grounds to discuss policy issues without explicit permission.

While Miller isn’t interested in acting as a gatekeeper for the president, he has told associates that he wants to limit the influence of corporate lobbyists on White House staff. The memo doesn’t completely limit all interactions with lobbyists.The move comes in the wake of allegations from some of Trump’s allies that people close to former White House Chief of Staff Susie Wiles cultivated cozy relationships with lobbyists and bent the administration agenda to the priorities of special interests.

The memo raised alarms across K Street, according to two longtime Republican lobbyists who said their firms are already developing strategies to comply with the stricter new rules.The turnover in the chief of staff’s office has also led to anxiety among aides worried about personnel changes, administration officials said. The White House moved quickly, for instance, to deny reports that Kevin Hassett, director of the National Economic Council, would step down in the coming months.Marco Rubio walks between presidential assistant Natalie Harp and Miller.One former Trump administration official said that the new chief had been taking stock of any executive actions that died on the vine during Wiles’s tenure, especially as they relate to Make America Healthy Again initiatives and issues that affect young men.

Other priorities Miller wants to tackle, people close to him say, include healthcare, urban crime and affordability.As Trump prepares to confront Democratic congressional majorities during his final two years in office, Russ Vought, director of the White House Office of Management and Budget, said Miller is both “a master of the intricacies of government” and “acutely aware that our most precious resource as an administration is time.”Previously a congressional aide to former Alabama Sen. Jeff Sessions, Miller comes to the job with deep relationships on Capitol Hill.

He quickly reached out to Republican congressional leaders, according to two sources familiar with his conversations, speaking directly with South Dakota Sen. John Thune, who will serve as minority leader, and Ohio Sen.

Bernie Moreno, who will lead Senate Republicans’ campaign arm.“He understands the president well,” Sen. Eric Schmitt (R., Mo.) said. “Stephen doesn’t need to be the main character, and views himself in a supporting role of this president and that’s why they’ve always had a good relationship.”Write to Meridith McGraw at Meridith.McGraw@WSJ.com, Philip Wegmann at philip.wegmann@wsj.com and Alex Leary at alex.leary@wsj.com