Stephen Miller, Donald Trump’s most controversial aide and the architect of his sweeping immigration crackdown, has lavished praise on the president after his appointment as White House chief of staff.

In his first public remarks since the move was announced, Miller – who begins the role on Monday – pledged “total devotion” to Trump amid concern the administration is now poised to double down on its far-right, nativist immigration agenda.

“If it wasn’t for President Trump the United States of America would not exist right now,” Miller, standing alongside his boss, declared at an event on Friday evening. “This man saved America. He saved our military from collapse. He saved our economy from destruction. He saved our border from annihilation. He saved our country from threats at home and abroad which would have been the end of the American dream for ever.”

His appointment was unveiled after Americans punished Trump’s Republican party in this week’s US midterm elections, unleashing a blue wave that swept through parts of the country where Democrats have not won in decades.

As of Saturday morning, Republicans have lost seven seats in the Senate, and Democrats hold narrow leads in the two races – Alaska and Mississippi – that have yet to be called. Republicans have also lost 42 seats in the House of Representatives, and five governor races.

Despite this extraordinary array of losses nationwide – which senior Republicans including JD Vance have blamed on the Iran war and tariffs, key pillars of Trump’s agenda – Miller praised the president on Friday for having “saved the Republican party from the ash heap of history and has made it stronger, more popular and more resilient than ever before”.

Miller added: “It is the honor of my life to be President Trump’s chief of staff, and I swear to you, and I pledge to you, Mr President, my absolute loyalty, my unwavering commitment, and my total devotion to your success and the success of this country.”

The speech will likely add to concerns that the Trump administration is set to continue pursuing its hardline immigration agenda, which Miller had been leading in his previous role as deputy chief of staff.

Miller’s appointment has been met with dismay by Democrats and some Republicans, with Marlin Stutzman, the Republican congressman from Indiana, describing the move as a “horrible decision” in comments to Politico.

Don Bacon, the outgoing Republican congressman from Nebraska, predicted that Miller will “have an antagonistic relationship with many”, telling CNN: “He cost us seats in Texas and Arizona by alienating many Hispanic voters who voted for Trump in 2024.”

Miller’s appointment was announced as Trump said he expects to make “a few changes” to his team.

“I love my cabinet. I think they’re good. But I’ll do – be doing maybe a few changes. I don’t want to mention who,” Trump told 60 Minutes, in an interview due to air on Sunday.

Asked if he has confidence in his defense secretary, Pete Hegseth, who has faced questions over his future in the role as the scale of the Republicans’ midterm losses became clear, Trump said “I do”, but notably stopped short of crediting him for what the president said were military successes.

Last month, after reports he might replace Susie Wiles as White House chief of staff, Trump was unequivocal: there is “zero chance” that will happen, he told reporters.