Acting White House Chief of Staff Mick Mulvaney filed a motion late Friday to join a lawsuit slated to test the House’s power to subpoena executive branch officials during the ongoing impeachment inquiry against President Trump.

Mulvaney, who did not show for his scheduled testimony Friday morning, is asking a federal court to decide whether he’s obligated to follow executive branch orders not to comply with the impeachment probe or follow through on a subpoena he received to testify before the House Intelligence Committee. His motion claims House Democrats threatened to hold him in contempt of Congress or pursue other actions against him for essentially following through with directives from his boss -- the president.

HOUSE WITHDRAWS SUBPOENA FOR FORMER NATIONAL SECURITY COUNCIL DEPUTY CHARLES KUPPERMAN

According to a copy of the motion obtained by Fox News, Mulvaney seeks to join as a plaintiff in a lawsuit filed by former White House Deputy National Security Adviser Charles Kupperman, who sued last month ahead of his scheduled testimony before the House Intelligence, Foreign Affairs and Oversight Committees.

“The question whether the President’s authority must give way in the face of a congressional subpoena — the determination Mr. Kupperman has asked this Court to make — is central to the question whether the House may take adverse action against Mr. Mulvaney, as threatened,” the motion said.

“The issues implicated by this motion are significant for the country generally and for Mr. Mulvaney personally. They go to the heart of our representative government and its promise to secure individual liberty by dividing the awesome power of government amongst itself,” the motion continued. “Mr. Mulvaney, like Mr. Kupperman, finds himself caught in that division, trapped between the commands of two of its co-equal branches — with one of those branches threatening him with contempt. He turns to this Court for aid and respectfully requests that his motion be granted.”

Kupperman, who left the administration when National Security Adviser John Bolton exited in September, also had asked the court to decide whether to follow the orders of the executive branch or Congress – which, under the Constitution’s separation of powers, are both equal branches of government. Mulvaney, on the other hand, is a current White House adviser.

House Democrats withdrew a subpoena for Kupperman’s testimony Wednesday, suggesting they would continue with the impeachment probe without hearing testimony from White House officials if they must. The federal court initially was slated to hear the case in mid-December, which would have derailed Kupperman from having to testify as part of the impeachment probe until his case is decided. The court's hearing on the case was expected to be canceled after House Dems withdrew his subpoena.

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House Democrats also issued a subpoena for Mulvaney this week, and he declined to show for his scheduled deposition Friday. It’s unclear how the lawsuit will proceed, pending if Mulvaney's motion to join as a plaintiff is approved, and whether House Democrats choose to withdraw the subpoena for his testimony.

Fox News’ James Levinson and Andrew O’Reilly contributed to this report.