President Donald Trump tapped Stephen Miller to succeed Susie Wiles as White House chief of staff, elevating an adviser who reportedly helped develop plans to advance his agenda without congressional approval as Democrats prepare to take control of Congress in January.

Miller, Office of Management and Budget Director Russell Vought and White House lawyers have worked since April to identify executive actions Trump could take without new legislation, including measures on immigration, tariffs, housing costs and federal spending, Reuters reported Sunday, citing three people familiar with the planning.

With Democrats set to control Congress in January, Trump will face greater obstacles to passing legislation, while Democratic lawmakers will gain the power to launch investigations into his administration.

"If crazed Democrats want to abuse their power and politicize their process of investigations, subpoenas, and destroying lives, they should be reminded that we still control the federal government and all of its powers," a White House official told Fox News Digital Friday afternoon.

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Asked by CBS on Tuesday how he would advance his agenda if Democrats took control of Congress, Trump cited the Supreme Court’s June ruling in Trump v. Slaughter, which expanded his authority to remove certain federal officials.

"We won the Slaughter case and the Slaughter case gives the president tremendous powers that it never had before," Trump told CBS in the interview.

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In Trump v. Slaughter, the Supreme Court handed Trump a significant victory on presidential authority in June, ruling that he had the constitutional power to remove former Federal Trade Commission Commissioner Rebecca Slaughter despite a federal law restricting the president's ability to fire FTC commissioners.

Trump said the ruling will be a cornerstone for how he intends to govern by executive action. He also discussed vetoing bills passed by the Democratic Congress until they're willing to come to the bargaining table. 

On numerous occasions, he has already referred to himself as "Mr. Veto."

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"As the President has said, he's willing to work with anyone to continue delivering victories for hardworking Americans," White House spokeswoman Lauren Bis told Fox News Digital. "If Democrats choose to pursue sham investigations and radical left policies, they are choosing to work against the American people."

Republicans on the Hill expressed concerns about what Miller's appointment could mean for the final two years of his presidency.

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Asked whether Miller's promotion could lead Trump to rely more heavily on executive action, one House Republican told Fox News Digital: "I’ve been concerned about that for two years."

"I’m also concerned [Trump] will embrace his populist roots that are not conservative to pass things with the Dems," the lawmaker said.

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Another said the Trump administration will continue governing "without Congress, as they've been doing."

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A Trump administration official praised Miller's appointment, particularly given Miller's immigration bona fides.

"A great pick," the official told Fox News Digital. "He has the trust of millions of Americans who understand that reversing mass migration is the crisis of our time."

Miller will begin his new role Monday, and Wiles will move to Trump's political operation.