(CNN) A key witness in the impeachment inquiry has sought for a court to decide if he should testify, caught between a House subpoena and a demand from the White House to protect President Donald Trump's immunity from testifying to Congress.

Charles Kupperman, who served until last month as deputy national security adviser at the White House, filed a lawsuit Friday asking a federal judge to rule on whether he is obliged to testify before House investigators. The case presents a new challenge to House Democrats seeking cooperation from close aides of Trump.

Kupperman, who worked closely with former national security adviser John Bolton, is scheduled to sit Monday for questioning by the three House committees handling impeachment. The lawsuit also raises questions about Bolton's possible future testimony , as Kupperman's lawyer Charles Cooper also represents Bolton.

"Plaintiff is faced with irreconcilable commands by the Legislative and Executive Branches of the Government and, accordingly, seeks a declaratory judgment from this Court as to whether he is lawfully obliged to comply with a subpoena issued by the House Defendants demanding his testimony '[p]ursuant to the House of Representatives' impeachment inquiry,' or he is lawfully obliged to abide by the assertion of immunity from congressional process made by the President in connection with the testimony sought from Plaintiff," the lawsuit states.

In a statement Saturday, Cooper reiterated that the lawsuit looked to have the judiciary, not Kupperman, resolve a conflict between the President and House Democrats.

"Under our system of Government, Constitutional disputes between the Legislative and Executive Branches should be adjudicated by the Judicial Branch, not by private citizens like Dr. Kupperman," Cooper wrote in a statement.

He added that "Dr. Kupperman takes no position on whether the command of the Legislative Branch or the command of the Executive Branch should prevail; he seeks only to carry out whichever constitutional obligation the Judicial Branch determines to be lawful and binding on him."

Kupperman was listening in on the July 25 phone call when, according to a White House transcript, Trump pressed Ukrainian President Volodymyr Zelensky to investigate former Vice President Joe Biden. The lawsuit comes following three weeks of administration officials testifying in House lawmakers' subsequent probe into the President's dealings with Ukraine.

The lawsuit includes a copy of a letter that White House counsel Pat Cipollone sent to Cooper on Friday directing Kupperman not to comply with the subpoena and maintaining that he would be protected by "constitutional immunity."

"The President, however, acting through the White House Counsel, has asserted that Plaintiff, as a close personal advisor to the President, is immune from Congressional process, and has instructed Plaintiff not to appear and testify in response to the House's subpoena," the lawsuit states.

Trump has slammed officials who have implicated him, such as the US' top diplomat in Ukraine, Bill Taylor. Taylor testified on Tuesday that he had been told Trump would withhold military aid to Ukraine until it publicly declared investigations would be launched that could help his reelection chances — including into Biden, according to a copy of Taylor's opening statement obtained by CNN.

But in the absence of a clear superseding power in the standoff between the White House's professed immunity and House Democrats' subpoena power, according to the lawsuit, Kupperman is asking the judge to decide.

"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," the challenge states.

This story has been updated.