The House committees investigating President Trump and Ukraine issued a subpoena Thursday night for acting White House Chief of Staff Mick Mulvaney to testify on Friday morning as part of their impeachment inquiry, two sources familiar tell Axios.

Why it matters: Mulvaney is the highest-ranking White House official to be subpoenaed yet, and the midnight-hour move suggests the committees are reaching into the final phase of their private investigation as they prepare to take their inquiry public next week.

The committees first subpoenaed Mulvaney to turn over documents in October, but subpoenaing him to appear for a hearing is a further escalation, and signals the committees are determined to hear him describe firsthand his role in the Ukraine saga.

Background: Several current and former Trump administration officials have told House investigators that Mulvaney carried out Trump’s directive to suspend $400 million in security assistance to Ukraine.

An official working on the impeachment inquiry, emphasized Mulvaney's Oct. 17 press briefing, during which he said the Trump administration froze military aid to pressure Ukraine to open a political investigation, as a reason for the subpoena. Mulvaney later walked those comments back.

emphasized Mulvaney's Oct. 17 press briefing, during which he said the Trump administration froze military aid to pressure Ukraine to open a political investigation, as a reason for the subpoena. Mulvaney later walked those comments back. "Other testimony during this inquiry also has indicated that Mr. Mulvaney could shed additional light on the President’s abuse of the power of his office for his personal gain," the official added.

The bottom line: It’s likely that Trump will exert executive privilege over his conversations with Mulvaney and argue that he has absolute immunity from complying with Congress’ requests — as he has done with other White House officials, such as Mulvaney aide Rob Blair and former deputy national security adviser Charles Kupperman.

Both Blair and Kupperman are waiting for the courts to determine whether they should comply with congressional subpoenas or White House orders blocking them from appearing.

The official working on the inquiry told Axios that Mulvaney "has the opportunity to uphold his oath to the nation and constitution by testifying tomorrow. We hope Mr. Mulvaney does not hide behind the President’s ongoing efforts to conceal the truth obstruct our investigation."

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