Ultra-loyalist Miller, 41, will replace Susie Wiles, who becomes the first major White House casualty from the Democratic landslide earlier this week.

Democrats pledged they would fight "every step of the way" against Miller, who became synonymous with scenes of masked immigration agents committing violence after he backed federal agents who shot and killed two Americans.

"I wanted to make a change," Trump told reporters at the White House when asked why he named Miller to replace Wiles.

"Sometimes in life you have to make moves."

Trump's move was particularly startling as he had repeatedly insisted before the midterms that there was "zero chance" of replacing Wiles.

A White House official told AFP that Wiles had resigned on Thursday night.

But naming the combative Miller was the clearest possible signal from Trump that the gloves are now off after his brief conciliatory gestures to Democrats in the wake of the Republican midterms collapse.

"He's a great fighter," Trump said later at the airport in West Palm Beach, Florida, where he was accompanied by Miller. "He's been with me right from the beginning...and he's very loyal."

Known for his sharp suits and sunglasses, Miller has been one of Trump's closest aides for decades, pushing nativist policies that gelled with Trump's "America First" agenda.

As Trump's deputy chief of staff for policy and homeland security advisor, he was the brains behind the mass deportation program that defined the first year of the president's second term.

But he faced widespread criticism after the deadly shootings of two protesters by immigration agents in Minneapolis in January – incidents that sparked mass demonstrations.

"Stephen Miller is the architect of the most barbaric, inhumane policies of this administration. Trump couldn't have made a worse choice," Minority Leader Chuck Schumer posted on X.

"Our Democratic majorities will fight him every step of the way."

‘It's Miller Time'

In a post on Truth Social earlier announcing Miller's appointment, Trump described him as "one of my most trusted advisors."

Trump said Wiles had "done an outstanding job" and would be "moving over to the Trump Political Operation. I want to thank her for her service as Chief of Staff – She has done a great job!"

White House communications director Steven Cheung said Miller was a "Trump OG, someone who has been with the President since the very beginning.

"He will help the Administration deliver on monumental victories for the rest of this historic second term."

Top administration figures also praised Wiles, the first female White House chief of staff, whom Trump dubbed the "ice maiden" for maintaining discipline in the West Wing.

For most of the last two years, the 69-year-old had largely prevented the chaotic staff movements and infighting that marred Trump's first term.

But as the midterms loomed the writing was increasingly on the wall for Wiles, who announced recently that she had been given the all-clear from breast cancer.

"Serving President Trump in the White House has been the honor of a lifetime,"  Wiles said in a statement on X, adding that Friday would be her last day in her job.

"I'm so proud to have been a part of what we’ve accomplished together from securing our border and cutting taxes to strengthening our economy and putting America First."

Major figures in Trump's Make America Great Again (MAGA) movement – reeling since the midterms – hailed Miller's appointment.

"It's Miller Time," posted right-wing journalist Jack Posobiec, referring to a famous slogan for an American beer brand.

Trump sat with CBS News program "60 Minutes" for an interview that airs Sunday, and in published excerpts he said he expects to make a "little bit" of additional staff changes.

"I'll [...] be doing maybe a few changes. I don't wanna mention who," he said.

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