House Democrats leading the 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 late Saturday threatened contempt proceedings against a former White House deputy national security adviser if he fails to appear for a scheduled deposition Monday.

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.), acting Oversight and Reform Committee 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 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 in a letter that Charles Kupperman’s lawsuit to prevent him from responding to a House subpoena was “without merit.”

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“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,” their letter states. “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.”

“Such willful defiance of a duly authorized subpoena may cause the committees to draw an adverse inference against the president,” they added. “The White House’s overbroad assertion of ‘absolute immunity,’ at its core, is another example of the president’s stonewalling of Congress and concerted efforts to obstruct the House’s impeachment inquiry.”

On Friday, Kupperman asked a federal judge to rule on whether he must appear, saying he faced the “irreconcilable demands” of both the House subpoena and the White House’s announcement that it will not cooperate with the impeachment inquiry.

“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,” his suit in the U.S. District Court for the District of Columbia reads.

The White House has invoked constitutional immunity to prevent Kupperman from testifying, going beyond executive privilege and asserting the person in question cannot be compelled to testify before Congress or appear at a hearing.