A top aide to former White House national security adviser 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 will not testify before Congress on Monday as part of the ongoing impeachment inquiry into 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, his lawyer has told Democratic investigators.

Charles Cooper, the attorney for former Bolton aide Charles Kupperman, said his client will not testify until a court rules on a lawsuit surrounding his appearance, according to a letter delivered Sunday to Democrats on the House Intelligence Committee.

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"If your clients' position on the merits of this issue is correct, it will prevail in court, and Dr. Kupperman, I assure you again, will comply with the Court's judgment," Cooper wrote to Daniel Noble, a Democratic attorney for committee.

Kupperman, the former deputy to Bolton, has been caught in a legal tug of war between a White House trying to block his testimony, citing executive privilege, and Democrats who have subpoenaed Kupperman to appear on Monday in the Capitol before the three House committees leading the impeachment probe.

On Friday, Kupperman filed a lawsuit, essentially asking the courts to rule on which side had the more compelling legal argument.

Democrats on the three committees fired back on Saturday, warning that Kupperman — now a private citizen — has a legal obligation to appear before Congress. A failure to do so, the committee chairs warned, could lead to charges of contempt of Congress.

“Notwithstanding this attempted obstruction, the duly authorized subpoena remains in full force and Dr. Kupperman remains legally obligated to appear for the deposition on Monday,” the letter by House Intelligence Committee Chairman Adam Schiff Adam Bennett SchiffSchiff to subpoena top DHS official, alleges whistleblower deposition is being stonewalled Schiff claims DHS is blocking whistleblower's access to records before testimony GOP lawmakers distance themselves from Trump comments on transfer of power MORE (D-Calif.), House Oversight and Reform Committee acting Chairwoman Carolyn Maloney Carolyn Bosher MaloneyPelosi, Democrats unveil bills to rein in alleged White House abuses of power Government watchdog recommends creation of White House cyber director position Top Democrats call for DOJ watchdog to probe Barr over possible 2020 election influence MORE (D-N.Y.) and House Foreign Affairs Committee Chairman Eliot Engel Eliot Lance EngelHouse panel halts contempt proceedings against Pompeo after documents turned over Engel subpoenas US global media chief Michael Pack The Hill's Morning Report - Sponsored by The Air Line Pilots Association - Pence lauds Harris as 'experienced debater'; Trump, Biden diverge over debate prep MORE (D-N.Y.) said.

“The deposition will begin on time and, should your client defy the subpoena, his absence will constitute evidence that may be used against him in a contempt proceeding."

Cooper, in his letter to Noble, said "it is not Dr. Kupperman who contests" the Democrats' legal claim.

"It is President Trump, and every President before him for at least the last half century, who have asserted testimonial immunity for their closest confidential advisors," Cooper wrote.

The White House successfully blocked former White House counsel Don McGahn from appearing before Congress pursuant to a subpoena earlier this year by asserting he is immune from compelled testimony. The developments have prompted a court fight.

The concept of "immunity" of top presidential advisers from congressional testimony has been invoked by both Republican and Democratic administrations, but there is little case law on the subject.

Kupperman was reportedly on the July 25 call between Trump and Ukrainian President Volodymyr Zelensky that is at the center of the impeachment investigation into allegations that Trump solicited foreign leaders to help his reelection in 2020.

Schiff said Sunday that Bolton is also a “very important” witness and House Democrats want him to testify. But he's already anticipating another standoff with the White House.

“My guess is they're going to fight us having John Bolton in,” Schiff said on ABC's “This Week.”

Morgan Chalfant contributed to this report, which was updated at 1:00 p.m.