Washington (CNN) House Democrats are threatening to hold Charles Kupperman in contempt after the former deputy national security adviser filed a lawsuit Friday asking a judge to decide whether he is obliged to appear for his deposition before House investigators.

House Intelligence Chairman Adam Schiff, Foreign Affairs Chairman Eliot Engel and acting Oversight Chairwoman Carolyn Maloney sent Kupperman a letter Saturday charging that his lawsuit was "lacking in legal merit" and coordinated with the White House, calling the effort "an obvious an 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."

"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 chairs wrote. "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."

The Kupperman 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.

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