Former White House deputy national security adviser Charles Kupperman filed a lawsuit on Friday, asking a D.C. federal court whether he should comply with a congressional subpoena to provide testimony as part of the House’s impeachment inquiry or follow President Trump’s direction to assert immunity.

Kupperman, who left the administration when ex-national security adviser John Bolton left his post last month, was slated to appear before the House Intelligence Committee, Foreign Affairs Committee, and Oversight Committee on Monday as part of their impeachment investigation into the Ukraine controversy.

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In the lawsuit filed Friday to the U.S. district court in Washington, Kupperman said he “cannot satisfy the competing demands of both the legislative and executive branches,” and said without the court’s help, he would have to make the decision himself, acknowledging that it could “inflict grave constitutional injury” on either Congress or the presidency.

At this point, it is unclear when the judge will rule on the case, and whether his congressional deposition will take place as scheduled.

Kupperman served as a deputy to Bolton, and the National Security Council’s current Russia and Europe director, Tim Morrison.

The lawsuit comes amid the formal impeachment inquiry into the president, which began after details emerged from his highly controversial July 25 phone call with Ukrainian President Volodymyr Zelensky, in which he pressured him to launch an investigation into former Vice President Joe Biden and his son Hunter Biden—specifically Hunter’s role on the board of a Ukrainian natural gas firm and the elder Biden's role in the ouster of a prosecutor looking into the firm's founder.

A whistleblower first complained about the contents of the phone call to the intelligence community inspector general in August, stating the call seemed to show the president soliciting a foreign power to help in influencing the 2020 presidential election by opening an investigation into his political opponent. This has since led to a parade of witnesses testifying behind closed doors about the president's effort to pressure Ukraine, including allegations that he held back military aid as leverage.

The White House and the president’s allies have maintained there was no such quid pro quo, though top diplomat Bill Taylor challenged such claims in testimony this week.

Meanwhile, on Saturday, acting assistant secretary of state for Europe Philip Reeker testified on Capitol Hill. Reeker was not directly involved in the debate over military aid to Ukraine. Current and former officials reportedly have said that the aid issue was instead given to U.S. ambassador to the European Union, Gordon Sondland, and Kurt Volker, the former U.S. envoy to Ukraine.

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As part of the House impeachment inquiry, Schiff, Foreign Affairs Committee Chair Eliot Engel, D-N.Y., and Oversight Committee acting Chair Carolyn Maloney, D-N.Y., on Friday also subpoenaed two Office of Management and Budget officials -- Associate Director for National Security Programs Michael Duffey and Acting Director Russel Voight -- for depositions next month.

The chairs on Friday also notified State Department Counselor T. Ulrich Brechbuhl of a subpoena compelling his testimony next month.

“Your failure or refusal to appear at the deposition, including at the direction or behest of the President or the White House, shall constitute evidence of obstruction of the House’s impeachment inquiry and may be used as an adverse inference against the President,” the Democratic lawmakers wrote in their letter to Brechbuhl.

Also expected to testify this week are director for national security affairs at the National Security Council Lt. Col. Alexander Vindman on Tuesday, Principal Deputy Assistant Secretary of Defense for International Security Affairs Kathryn Wheelbarger on Wednesday and Morrison on Wednesday.

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Morrison’s upcoming interview will be especially critical to the impeachment inquiry, after Taylor’s damning testimony last week.

Taylor testified that he had a phone call with Morrison, who told him that Sondland told top Ukrainian aide Andriy Yermak “that security assistance money would not come until President Zelensky committed to pursue the Burisma investigation.”

Burisma Holdings is the Ukrainian natural gas firm where Hunter Biden was employed in a lucrative role on the board.

“I was alarmed by what Mr. Morrison told me about the Sondland-Yermak conversation. This was the first time I had heard that the security assistance not just the White House meeting—was conditioned on the investigations,” Taylor testified.

Meanwhile, Trump has maintained that he has done nothing wrong, and is not concerned about the impeachment inquiry.

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"The Fake Washington Post keeps doing phony stories, with zero sources, that I am concerned with the Impeachment scam. I am not because I did nothing wrong. It is the other side, including Schiff and his made up story, that are concerned. Witch Hunt continues!" he tweeted.

The Associated Press contributed to this report.