Acting White House chief of staff Mick Mulvaney Mick MulvaneyOn The Money: House panel pulls Powell into partisan battles | New York considers hiking taxes on the rich | Treasury: Trump's payroll tax deferral won't hurt Social Security Blockchain trade group names Mick Mulvaney to board Mick Mulvaney to start hedge fund MORE asked a court on Friday to allow him to join a lawsuit testing the House’s ability to subpoena him in its impeachment investigation into President Trump Donald John TrumpFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Former Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick MORE.

Mulvaney skipped a scheduled appearance on Friday to testify behind closed doors on Capitol Hill, claiming he was legally immune after the White House told him to not appear. Two witnesses whose testimony was publicized Friday said that he was a key player in trying to pressure Ukraine to open investigations that would be politically helpful to Trump.

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Later Friday evening, Mulvaney asked a federal court to allow him to join a lawsuit to provide guidance on the House’s powers, saying he cannot choose between the White House’s instruction and the House’s subpoena.

Mulvaney’s lawyers wrote that the questions raised in the lawsuit "are significant for the country generally and for Mr. Mulvaney personally" and "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," they added.

Mulvaney is asking to be added to a lawsuit filed last month by Charles Kupperman, who served as Trump’s deputy national security adviser and was subpoenaed by House Democrats last month to testify in their investigation into the president’s dealings with Ukraine.

“Plaintiff obviously cannot satisfy the competing demands of both the legislative and executive branches, and he is aware of no controlling judicial authority definitively establishing which branch’s command should prevail,” Kupperman’s suit to the U.S. District Court for the District of Columbia reads.

Mulvaney added in his filing that protection of his interactions with Trump are even more important given his role in the White House.

"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 has emerged as a focal figure in the House’s impeachment investigation after he confirmed during a press conference last month that Trump froze nearly $400 million in military aid to Ukraine to pressure the country into investigating former Vice President Joe Biden Joe BidenFormer Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick Bloomberg rolls out M ad buy to boost Biden in Florida MORE, a chief 2020 political rival, though he backtracked hours later.