A top aide to former White House national security adviser John BoltonJohn BoltonNSA places former GOP political operative in top lawyer position after Pentagon chief's reported order After insurrection: The national security implications McConnell won't reprise role as chief Trump defender MORE will not testify before Congress on Monday as part of the ongoing impeachment inquiry into President Trump
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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.
"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 SchiffAdam Bennett SchiffAngus King warns of 'grave danger' of Trump revealing classified information Schiff says 'massive intelligence and security failure' led to Capitol breach Sunday shows - Capital locked down ahead of Biden's inauguration MORE (D-Calif.), House Oversight and Reform Committee acting Chairwoman Carolyn Maloney
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“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.

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.