An ex-White House adviser who’s supposed to testify before House impeachment investigators on Monday has asked a federal court whether he should comply with a subpoena or follow President Donald Trump’s directive against cooperating in what the president dubs a “scam.”

After getting a subpoena Friday, former deputy national security adviser Charles Kupperman quickly filed a lawsuit in U.S. district court in Washington. He asked a judge to decide whether he should accede to House demands for his testimony or to assert “immunity from congressional process” as directed by Trump.

The lawsuit came as Democrats’ impeachment inquiry continued at full speed with a rare Saturday session. Philip Reeker, the acting assistant secretary of state for Europe, took questions behind closed doors for more than eight hours about Trump’s ouster of the ambassador of Ukraine in May and whether he had knowledge about efforts to persuade Ukraine to pursue politically motivated investigations.

Kupperman, who provided foreign policy advice to the president, was scheduled to testify in a similar session on Monday. In the lawsuit, Kupperman said he “cannot satisfy the competing demands of both the legislative and executive branches.” Without the court’s help, he said, he would have to make the decision himself — one that could “inflict grave constitutional injury” on either Congress or the presidency.

The impeachment inquiry is rooted in a July 25 phone call Trump made to Ukrainian President Volodymyr Zelenskiy. During the call, Trump asked the Ukrainian leader to pursue investigations of Democratic political rival Joe Biden’s family and Ukraine’s role in the 2016 election that propelled Trump into the White House.

At the time of the call, Trump was withholding congressionally approved military aid for Ukraine. He has repeatedly said there was no quid pro quo for the Ukraine investigations he was seeking, though witness testimony has contradicted that claim.

Kupperman’s filing says “an erroneous judgment to abide by the President’s assertion of testimonial immunity would unlawfully impede the House from carrying out one of its most important core Constitutional responsibilities” — the power of impeachment — and subject Kupperman to “potential criminal liability for contempt of Congress.”