U.S. House Intelligence Committee Chairman Rep. Adam Schiff (D-CA) speaks to members of the media outside a closed session before the House Intelligence, Foreign Affairs and Oversight committees at the U.S. Capitol on October 28, 2019 in Washington, DC. Also pictured are (L-R) Rep. Carolyn Maloney (D-NY), Rep. Jamie Raskin (D-MD) and Rep. Eric Swalwell (D-CA).

Schiff, D-Calif., said he believes that Kupperman's testimony before the House Intelligence, Oversight and Foreign Affairs committees "would corroborate the allegations of misconduct that other witnesses have made."

"Witnesses like Dr. Kupperman need to do their duty and show up," Schiff told reporters after Kupperman failed to appear for a scheduled deposition.

House Intelligence Chairman Adam Schiff said Monday that former national security adviser John Bolton's deputy Charles Kupperman may be held in contempt after he failed to comply with a subpoena to testify in the impeachment inquiry into President Donald Trump.

Schiff, who has been leading the impeachment probe in the House, also said the panels will "obviously consider, as we inform Dr. Kupperman's counsel, his failure to appear as evidence that may warrant a contempt proceeding against him."

The White House has vowed not to cooperate with the inquiry, which Trump has regularly decried as a "witch hunt." Kupperman filed a lawsuit Friday asking a judge to rule whether he must comply with the Trump administration or with the congressional subpoena for his testimony.

Ahead of Schiff's remarks, Republican Rep. Jim Jordan told reporters that Kupperman is "more than willing to come" if the court rules that he must comply with the subpoena. "But obviously he's not coming today," said Jordan, the ranking Republican on the Oversight Committee.

Schiff said the White House's effort to block Kupperman from talking to the three House panels conducting the inquiry suggest that his testimony would bolster their case.

"I think we can infer from the White House opposition to Dr. Kupperman's testimony that they believe that his testimony would be incriminating of the president," Schiff said. "It is also, I think, very plain, additional and powerful evidence of obstruction of Congress and its lawful function by the president."

"If this witness had something to say that would be helpful to the White House, they would want him to come and testify," Schiff added. "They plainly don't."

The impeachment inquiry into Trump began in earnest shortly after a CIA whistleblower raised concerns about Trump's July 25 call with Ukraine President Volodymyr Zelensky, in which the U.S. president asked Ukraine to "look into" unsubstantiated corruption allegations against former Vice President Joe Biden and his son Hunter. Trump also asked Zelensky in that call to "do us a favor though" and look into Ukraine's alleged role in foreign interference in the 2016 presidential election.

Democrats say they have already heard "damning" testimony against Trump in the impeachment inquiry. Acting U.S. ambassador to Ukraine Bill Taylor, for instance, testified last week that it was made clear to him that a military aide package to Ukraine was contingent upon that country announcing probes into the Bidens and the 2016 election.

″'Everything' was dependent on such an announcement, including security assistance," Taylor quoted U.S. Ambassador Gordon Sondland as saying. Sondland, ambassador to the EU, had been deposed earlier in the inquiry. He returned to Capitol Hill on Monday to review his testimony, Jordan said.

Kupperman had served as deputy national security advisor under Bolton, whom Trump says he fired in September. Bolton maintains that he "offered to resign." Kupperman briefly served as acting national security advisor before Trump selected Robert O'Brien for the role.

Kupperman was reportedly listening in on the July 25 call, a partial transcript of which was released by the White House just after House Speaker Nancy Pelosi announced the impeachment inquiry.