U.S. President Donald Trump’s former Deputy National Security Advisor Charles Kupperman defied a subpoena issued by House Democrats compelling him to testify Monday in the ongoing impeachment probe.

Kupperman’s decision has dismayed House Democrat investigators, who have threatened to hold him in contempt of Congress. In a letter to Kupperman’s lawyer issued Saturday, the House Democrat leaders cautioned that his absence may prompt them to “draw adverse inference” that his testimony would have fueled their impeachment agenda.

In other words, Democrats may assume Kupperman would have said something against the president if he does not testify.

Democrats have cautioned witnesses and agencies that defying subpoenas amounts to obstruction of Congress, which they could cite in articles of impeachment. Kupperman, however, is reportedly the first impeachment probe witness to be threatened with contempt for not appearing.

House Intelligence Committee Chairman Adam Schiff (D-CA) lamented to reporters Monday that it was “deeply regrettable” that Kupperman was a “no-show.”

“He was compelled to appear with a lawful congressional subpoena,” Schiff added. “Witnesses like Dr. Kupperman need to do their duty and show up.”

Jim Jordan from Ohio, the top Republican on the House Committee on Oversight and Reform, confirmed that Kupperman would wait to testify until a judge rules on the congressional subpoena compelling him to do so.

“He’s waiting on the court to rule … if the court says he has to come, he’s more than willing to come,” Jordan told reporters on Capitol Hill.

Charles Cooper, Kupperman’s attorney, told House Democrat investigators in a letter delivered Sunday, that his client would not give a deposition until a court rules on a lawsuit surrounding his appearance, the Hill noted.

Kupperman filed a lawsuit Friday essentially asking the courts to rule on whether he should testify.

The former top aide to former White House national security adviser John Bolton is worried that his testimony would “inflict grave Constitutional injury on either the House or the President,” USA Today reported, adding:

Kupperman noted in the 17-page filing that if he defies Trump, he could hurt the president’s ability to receive confidential advice from top aides. But if he defies the House subpoena, he could impede their constitutional duty to investigate potential impeachment and could be subject to criminal penalties for contempt. Kupperman said he didn’t take a position on whether the executive or legislative branch should prevail, but that the judicial branch should resolve the dispute.

In the letter to Democrat investigators, Cooper declared:

We want to assure your clients, again, that it is not Dr. Kupperman who contests your clients’ constitutional claim. It is President Trump. 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.

House Democrat chairman pursuing the impeachment probe — from the Committees on Oversight and Reform, Intelligence, and Foreign Affairs — are seemingly fuming over Kupperman’s refusal to testify.

Cooper’s correspondence Sunday came in response to a letter issued Saturday by House Democrat leaders, threatening to hold Kupperman in contempt.

In the letter sent to Kupperman’s counsels Saturday, the chairmen noted:

Kupperman’s lawsuit—lacking in legal merit and apparently coordinated with the White House—is an obvious and 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 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. In light of the direction from the White House, which lacks any valid legal basis, the Committees shall consider your client’s defiance of a congressional subpoena as additional evidence of the president’s obstruction of the House’s impeachment inquiry. Such willful defiance of a duly authorized subpoena may cause the Committees to draw an adverse inference against the President, including that your client’s testimony would have corroborated other evidence gathered by the Committees showing that the President abused the power of his office by attempting to press another nation to assist his own personal political interests and not the national interest.

The White House has said it will not cooperate in the impeachment inquiry because it is “illegitimate” until the full House votes on the matter and unfair.

Kupperman was reportedly on the July 25 call between Trump and Ukrainian President Volodymyr Zelensky that triggered the impeachment probe into claims that Trump asked for help in his reelection in 2020, the Hill noted.

Bolton is reportedly considering testifying in the impeachment probe.