House impeachment investigators have requested acting White House chief of staff Mick Mulvaney testify on Capitol Hill on Friday, ratcheting up their investigation to target the President's top aide.

The House Intelligence, Foreign Affairs and Oversight committees sent Mulvaney a letter Tuesday requesting he appear for a closed-door deposition as part of the Democrats' impeachment inquiry into President Donald Trump and Ukraine.

"We believe that you possess substantial first-hand knowledge and information relevant to the House's impeachment inquiry," the Democratic chairs wrote.

It appears unlikely that Mulvaney will comply with the request.

So far this week, six Trump administration officials have defied requests for their testimony, and several more are expected to do the same later this week, including Energy Secretary Rick Perry and acting Office of Management and Budget Director Russ Vought.

Mulvaney and the White House have already failed to respond to a subpoena from the Democrats for documents, and White House counsel Pat Cipollone sent House Democrats a lengthy letter arguing that the impeachment inquiry was illegitimate.