Charles Kupperman, who served as a deputy to former national security adviser John Bolton, did not show up to testify on Capitol Hill on Monday — despite a congressional subpoena and a warning from Democrats that a failure to appear could result in a contempt citation.

On Friday, Kupperman filed a lawsuit asking a federal judge to resolve conflicting orders from Congress and the White House, which had instructed him not to appear. Over the weekend, his attorney Charles Cooper reiterated Kupperman’s desire to have the courts resolve the dispute before he appears. 

The case could become the major test of a constitutional dispute between the Trump administration and Congress. The White House has sought to prevent present and former advisers from testifying on the Ukraine matter and other topics, offering a broad resistance to cooperating with Congress on a wide range of matters.

Attorneys for Kupperman have asked the courts for expedited consideration of their lawsuit.

On Saturday, three House Democratic committee chairs sent Cooper a letter arguing the lawsuit lacked merit and appeared to have been coordinated with the White House. House Intelligence Committee Chairman Adam B. Schiff (D-Calif.), Foreign Affairs Committee Chairman Eliot L. Engel (D-N.Y.) and acting Oversight Committee Chairwoman Carolyn B. Maloney (D-N.Y.) called Kupperman’s suit “an obvious and desperate tactic by the president to delay and obstruct the lawful constitutional functions of Congress and conceal evidence about his conduct from the impeachment inquiry.” 

Cooper responded with a letter of his own Saturday night, saying the lawsuit had not been “even discussed” with anyone at the White House. 

He also argued that the courts need to provide clarity on this dispute between two branches of government. “It would not be appropriate for a private citizen, like Dr. Kupperman, to unilaterally resolve this momentous constitutional dispute between the two political branches of our government,” wrote Cooper, who is also representing Bolton. 

The conflict continued into Sunday evening, when the three committee chairs again insisted Kupperman appear Monday. In response to Cooper’s insistence on deferring to the courts, they wrote he “has a simple choice to make: either appear for a deposition tomorrow pursuant to a duly authorized subpoena, or abide by a baseless White House assertion that your client, a private citizen, should disregard his own legal obligations.”

Congressional Democrats have been eager to talk with both men, who are expected to be able to shed light on internal White House discussions about Ukraine. The pair are longtime political appointees, having worked in previous GOP administrations and on presidential campaigns.

House Democrats subpoenaed Kupperman on Friday, ordering him to appear for a previously scheduled Monday morning deposition at the Capitol.

Kupperman’s attorney immediately asked the White House for President Trump’s position on the subpoena. In a letter, White House counsel Pat Cipollone responded that “constitutional immunity” protected Trump’s current and former senior advisers and that Kupperman was “absolutely immune from compelled congressional testimony with respect to matters related to his service as a senior adviser to the President.”

The letter ordered Kupperman not to comply with the subpoena.

The directives have created a deep dilemma for Kupperman, according to the lawsuit: He wants to do his constitutional duty but needs the courts to step in to tell him exactly what that is.

“Absent a definitive judgment from the Judicial Branch,” the lawsuit said, “Plaintiff will effectively be forced to adjudicate the constitutional dispute himself, and if he judges wrongly, he will inflict grave constitutional injury on either the House or the President.”

The lawsuit said that the Justice Department’s Office of Legal Counsel has held for nearly 50 years that the president and his immediate advisers can’t be compelled to testify on matters related to their official duties. 

But the filing also cited a 2008 case, related to the George W. Bush administration’s controversial termination of nine U.S. attorneys, in which a federal court ruled that “absolute immunity” had some limits. In that case, the court found that the president’s counsel was not entitled to absolute or qualified immunity in a congressional inquiry. 

The lawsuit names Trump and several top House Democrats, including House Speaker Nancy Pelosi (D-Calif.) and Schiff (D-Calif.), who is leading the inquiry.

If Kupperman did show up and testify, the constitutional issue would be effectively mooted, Cooper wrote Saturday. The proper course, he said, “is to lay the conflicting positions before the court and abide by the court’s judgment.”

Carol D. Leonnig contributed to this report.