President Donald Trump has instructed the Justice Department to investigate Democrats, accusing them of threatening members of his cabinet and his supporters days after the party took control of both houses of Congress.

In a social media post on Saturday, Trump called the Democrats' conduct criminal and described it as extortion of the highest order, but gave no evidence. He said some of his cabinet had been warned of impeachment and claimed, again without proof, that Democrats were targeting his donors for their political views. He singled out Senator Elissa Slotkin of Michigan.

Since the midterm results, Democratic lawmakers and incoming members have signalled investigations into the Trump family's business dealings, immigration enforcement, Defence Secretary Pete Hegseth and Attorney General Todd Blanche. Once the new Congress sits in January, its committees will be able to demand documents, compel officials to testify and hold public hearings.

Representative Robert Garcia of California, expected to lead the House Oversight Committee, the chamber's main investigative panel, responded to the result with a blunt online warning to administration figures to hire lawyers.

During his second term Trump has repeatedly called for inquiries into opponents, among them former FBI director James Comey and New York Attorney General Letitia James.