Washington — Acting White House chief of staff Mick Mulvaney is seeking to join a lawsuit asking a federal court to decide whether a White House official should either comply with a congressional subpoena in the impeachment inquiry or follow a White House directive not to testify.

President Trump is among the co-defendants in the suit, raising the prospect of the president's own chief of staff joining a lawsuit against him.

The suit was filed by Charles Kupperman, a former deputy national security adviser under John Bolton. An attorney for Mulvaney noted in a motion to intervene in the case that the House was threatening to hold his client in contempt for defying a subpoena to appear, in spite of his "unquestioned status as a close and senior advisor to the President."

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It's a question, Mulvaney attorney William Pittard said in the filing, of "whether the President's authority must give way in the face of a congressional subpoena," as well as whether the House "may take adverse action against" Mulvaney for defying its subpoena.

The House has withdrawn the subpoena against Kupperman and said that he should adhere to the ruling in a separate case over a subpoena for former White House counsel Don McGahn, a case that has been underway since early August. House Democrats sued McGahn for his testimony regarding "the scope and extent of misconduct by President Trump." That case is likely to be decided sooner than Kupperman's case.

In October, Kupperman, who had been subpoenaed by committees conducting the impeachment inquiry, was ordered by the White House not to appear.

Bolton, who was not subpoenaed by the House, has already said he would abide by the decision in this case, but Mulvaney's new attempt to join Kupperman's case as a plaintiff is of interest because he is explicitly comparing himself to Kupperman. The judge will now have to decide whether Mulvaney and Kupperman hold comparable roles in the White House and are thus able to take similar stances in the lawsuit.

Pittard made several arguments in the filing that Mulvaney should not be forced to testify. He said the Justice Department Office of Legal Counsel, Pittard said, "consistently has concluded that close personal advisors of presidents are immune from compelled congressional testimony."

U.S. District Court Judge Richard Leon said he would hold a telephone conference regarding the motion by Mulvaney on Monday. The hearing is being held by phone because the courts are closed on Veterans Day.

An official working on the impeachment inquiry told CBS News that the president's directive to Mulvaney to defy the congressional subpoena "has added to the growing body of evidence documenting his attempts to obstruct the House's impeachment inquiry."

The official also said, "If Mr. Mulvaney had information that contradicts the consistent and incriminating testimony of numerous public servants, Mr. Mulvaney would be eager to testify, instead of hiding behind the President's ongoing efforts to conceal the truth."

Reporting by Rob Legare and Clare Hymes. Margaret Brennan contributed to this report.