See more of the story

– The House Democrats leading the impeachment inquiry of President Donald Trump demanded that a former White House official appear for his scheduled hearing Monday, dismissing his attempt to challenge a subpoena in court.

Charles Kupperman, the president's former deputy national security adviser, has taken the extraordinary step of asking a federal court whom he should listen to — Congress or Trump.

In a letter to attorneys representing Kupperman, leaders of three House committees said his lawsuit is "lacking in legal merit and apparently coordinated with the White House," and failure to appear for his deposition "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," the letter said. "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."

The episode is the most recent example of the constitutional clash that has slowed congressional oversight of Trump, adding to the legal tussle between two branches of government. The committees on Intelligence, Oversight and Foreign Affairs have issued subpoenas to most of the impeachment witnesses heard this month to give them legal cover to defy White House orders to not cooperate.

The letter was sent by Intelligence Committee Chairman Adam Schiff, Foreign Affairs Committee Chairman Eliot Engel and acting Oversight Committee Chairwoman Carolyn Maloney.

Kupperman on Friday made the argument that he was caught between two different directives and asked a federal judge whether he must appear. He said in court papers that he faces "irreconcilable commands" — a subpoena from House Democrats requiring him to cooperate and an order from the White House not to testify.

His lawyer, Charles Cooper, said that Trump "has asserted that Dr. Kupperman, as a close personal adviser to the president, is immune from Congressional process, and has instructed Dr. Kupperman not to appear and testify in response to the House's subpoena."

Besides Kupperman, Democrats have scheduled other national security officials to testify this week behind closed doors — an attempt to get a better look inside the White House as Trump pushed Ukraine to conduct politically motivated investigations.

The officials also include former National Security Adviser John Bolton and current NSC staffers Tim Morrison and Alexander Vindman. Morrison is particularly significant. William Taylor, the top U.S. diplomat in Ukraine, told lawmakers in his deposition last week about phone calls he had with Morrison that described the Ukraine effort.

Schiff, who is leading the inquiry, said Sunday he would like Bolton to testify but expects the White House would "fight us" over his appearance. Bolton is "a very important witness" who has "very relevant information," Schiff, D-Calif., told ABC's "This Week."

The committees are scheduled to hear from three other State Department and Defense Department witnesses, as well. Lawmakers want to determine whether military aid to Ukraine was held up as a condition of Ukraine's agreement to announce an investigation into former Vice President Joe Biden and his son Hunter.

The Democrats are moving quickly, sometimes scheduling multiple depositions in one day. They're trying to compile facts and eventually transition to public hearings. Schiff said Saturday that the committees are making "rapid progress."

A judge on Friday ordered the Justice Department to give the House secret grand jury testimony from special counsel Robert Mueller's Russia investigation, handing a victory to Democrats who want the material for the impeachment inquiry.