Acting White House chief of staff Mick Mulvaney on Friday asked to join a federal lawsuit seeking a judicial ruling on whether Congress can compel President Trump’s senior advisers to testify in the impeachment inquiry.

The lawsuit was originally filed late last month by Charles Kupperman, a former top national security aide to Trump, who said he faced conflicting orders from House Democrats and the White House over whether he must participate in the investigation and needed the court to tell him what his constitutional duty was.

Attorneys for Mulvaney said the acting chief of staff was facing the same dilemma.

Democrats on the House Intelligence Committee subpoenaed Mulvaney earlier this week and threatened to hold him in contempt if he refused to comply. In response, White House counsel Pat Cipollone instructed him not to testify, saying Mulvaney, who skipped his scheduled deposition Friday morning, was protected by “constitutional immunity” that extended to all of Trump’s current and former senior advisers.

The questions raised in the case “go to the heart of our representative government and its promise to secure individual liberty by dividing the awesome power of government amongst itself,” Mulvaney’s attorneys, Christopher Muha and William Pittard, wrote in the filing.

“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,” they wrote. “He turns to this Court for aid.”

Mulvaney’s request, if granted, would add further weight to a lawsuit that could have far-reaching effects on the House’s inquiry into the president’s efforts to pressure Ukraine into investigating his political rivals.

Democrats are keen on hearing from Mulvaney, who has emerged as a central player in consequential decisions involving Ukraine. Mulvaney effectively admitted in a news conference last month that the administration withheld U.S. military aid to Ukraine to muscle the country’s leaders into launching investigations that could help Trump politically. He later tried to walk back the remarks, but Democrats consider them to be a key piece of evidence in their impeachment case.

House investigators viewed Kupperman, who served as a deputy to former national security adviser John Bolton, as an important witness as well. Both he and Bolton had access to private deliberations at the White House involving the president’s communications about Ukraine.

But on Wednesday, House lawyers said they had withdrawn Kupperman’s subpoena to avoid a delay in the impeachment proceedings. They asked a federal judge to dismiss the case, saying they would instead look for guidance in the outcome of similar litigation involving a subpoena to former White House counsel Donald McGahn.

Responding to the dismissal request, an attorney representing Kupperman and Bolton, Charles Cooper, wrote in a letter to House lawyers that neither of the men would participate in the inquiry until a judge rules on whether they are legally permitted to testify. “It is important both to Dr. Kupperman and to Ambassador Bolton to get a definitive judgment from the Judicial Branch determining their Constitutional duty in the face of conflicting demands of the Legislative and Executive Branches,” Cooper wrote in the letter, which was filed with the court Friday.

Like Kupperman, Mulvaney received a written directive from Cipollone saying the president was ordering him not to appear for his deposition in the House. Cipollone wrote that the Department of Justice’s Office of Legal Counsel had determined that the president and his immediate advisers were “absolutely immune” from compelled congressional testimony — a position the department has taken in previous administrations.

Mulvaney’s attorneys said he should be included in Kupperman’s lawsuit not just because the two men were facing almost identical competing demands, but because Mulvaney was still a member of the administration, unlike Kupperman, and therefore still a subordinate of Trump’s.

“Mr. Mulvaney is both a closer and a more senior adviser to the President than was Mr. Kupperman,” the filing read. “In short, there are reasons unique to Mr. Mulvaney’s position that might form the basis of a judicial ruling against the House Defendants’ threatened actions, reasons that Mr. Kupperman necessarily cannot advance.”

The lawsuit names Trump and several top House Democrats, including House Intelligence Committee Chairman Adam B. Schiff (D-Calif.), who is leading the inquiry.

A spokesman for the Intelligence Committee did not immediately respond to a request for comment.