A key witness in the House’s impeachment investigation filed a lawsuit in federal court Friday asking a judge to rule on whether he can testify to congressional investigators after 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 invoked “constitutional immunity."

Charles Kupperman, who served as President Trump’s deputy national security adviser, was subpoenaed by House Democrats on Monday to testify in their investigation into the president’s dealings with Ukraine. However, the White House declared Friday that Trump had invoked “constitutional immunity” to bar Kupperman from appearing on Capitol Hill.

ADVERTISEMENT

“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 suit to the U.S. District Court for the District of Columbia reads.

Democrats hope that Kupperman and John Bolton John BoltonJudge appears skeptical of Bolton's defense of publishing book without White House approval Maximum pressure is keeping US troops in Iraq and Syria Woodward book trails Bolton, Mary Trump in first-week sales MORE, former national security adviser, can deliver key pieces of information in their probe. Both were close advisers to the president, dealing with him directly on policy toward Ukraine, and could give investigators insight into closed-door conversations.

Constitutional immunity extends beyond the powers of executive privilege, which the White House has used to block testimony from several other current and former officials. While executive privilege frees a person from being forced to divulge any dealings with the White House to investigators, constitutional immunity means individuals cannot be forced to testify before Congress at all, and do not even have to appear at hearings.

“The president … has asserted that plaintiff, as a close personal adviser 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 reads.

Kupperman “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 … or he is lawfully obliged to abide by the assertion of immunity from congressional process made by the president,” it continues.

Trump’s dealings with Ukraine are at the heart of the House’s impeachment investigation, with Democrats accusing the president of withholding $400 million in military aid to pressure Ukrainian President Volodymyr Zelensky 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 political rival.

Trump has denied any wrongdoing, calling a July call with Zelensky — in which he repeatedly pushed him to launch a probe into Biden — “perfect,” and denying that the halted military aid was tied to his request to investigate Biden.

However, William Taylor, who serves as the chargé d’affaires for Ukraine, testified Tuesday that he believed there was a quid pro quo.

“During our call on September 8, Ambassador Sondland tried to explain to me that President Trump is a businessman," Taylor told congressional investigators during his nearly 10-hour appearance behind closed doors, referring to Ambassador to the E.U. Gordon Sondland. "When a businessman is about to sign a check to someone who owes him something, he said, the businessman asks that person to pay up before singing the check."