Washington (CNN) Acting White House chief of staff Mick Mulvaney is attempting to join a lawsuit testing House subpoena power, which, if allowed, could effectively derail him from giving testimony in the impeachment inquiry until a federal court decides the case.

Mulvaney did not appear for his scheduled testimony Friday morning despite him receiving a subpoena, because the White House told him he should not appear, claiming that legally he is immune. According to transcripts of two other witness' testimony released Friday, Mulvaney was a key broker inside the White House pushing for Ukraine's announcement of investigations that could politically help President Donald Trump.

U.S. District Court Judge Richard Leon has scheduled a hearing in Washington, DC, for Monday at 5 p.m. to hear arguments about Mulvaney's request to join the already filed lawsuit.

On Friday night, Mulvaney asked a court to allow him to join a lawsuit about the House subpoena power over White House witnesses in the impeachment probe. Mulvaney wrote he fears the House could hold him in contempt, while he obeys executive branch orders.

He claims that he cannot choose between complying with the House subpoena he received and the White House's instruction that he is immune from testifying, and would like a federal court to decide for him.

An official working on the impeachment inquiry said Saturday that Mulvaney's effort to join the lawsuit to test the House's subpoena power would not delay the proceedings.

"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. As we have said many times, we will not allow this President, and those who seek to protect him at all costs, to play rope-a-dope in the courts with the impeachment inquiry," an official working on the impeachment inquiry said in a statement to CNN.

The official added that the White House directing Mulvaney to defy the congressional subpoena to appear on Friday for a deposition "has added to the growing body of evidence documenting his attempts to obstruct the House's impeachment inquiry."

The White House has not responded to a request for comment in reaction to the statement.

Mulvaney's move in court adds him to the ongoing case of another uncooperative impeachment witness, former deputy national security adviser Charles Kupperman , who sued House members and the White House before his scheduled testimony, asking a court to resolve whether he should follow the White House's orders that he is immune from testimony or the House's subpoena. Kupperman's case isn't set to be resolved for weeks, if not until mid-December or later.

Kupperman's team was not consulted in advance of Mulvaney's court filing saying he wanted to join in Kupperman's ongoing lawsuit regarding the authority of House subpoenas, a source familiar with the matter confirmed to CNN. The source also said the team was not given advanced knowledge. It was unclear whether Kupperman will object to the move by Mulvaney before Monday's planned court hearing. The New York Times first reported this development.

The questions Mulvaney raises with his proposed lawsuit Friday "are significant for the country generally and for Mr. Mulvaney personally," his lawyers wrote in the late-night filing. "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.

"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."

Mulvaney also notes he's in an especially powerful and unique role within the White House, making protection of his interactions with the President even more necessary for the court to weigh.

"Both in his capacity as the Acting White House Chief of Staff and as the Director of the Office of Management and Budget, (Mulvaney) met with and advised President Trump directly on a frequent and regular basis and implemented President Trump's plans," his attorneys wrote.

Mulvaney confirmed during a White House press briefing last month that Trump froze nearly $400 million in US security aid to Ukraine partially to pressure the country into investigating Democrats. Hours later, he denied having said so.

Kupperman, whose lawsuit Mulvaney wants to join, is using the same defense lawyer as former national security adviser John Bolton, who did not appear for his scheduled testimony Thursday. While those two share an attorney, Mulvaney has a different lawyer.

Mulvaney's approach Friday marks a growing effort by top White House officials to stonewall the House and test their legal arguments in court as the House nears public testimony from witnesses regarding impeaching the President.

Historically, the White House and Congress had often reached deals to resolve their standoffs without the court's intervention. In recent cases related to the House impeachment inquiry, federal judges have repeatedly sided with the House, saying it should get what it has asked for. But executive branch proponents are still attempting to keep some matters, like those involved in national security, confidential even in congressional investigations.