A former White House official on Monday told the House Intelligence Committee he would not testify in the congressional impeachment probe 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 given the White House's argument of immunity.

Charles Kupperman, the former deputy to former White House national security adviser John Bolton John BoltonJudge appears skeptical of Bolton's defense of publishing book without White House approval Maximum pressure is keeping US troops in Iraq and Syria Woodward book trails Bolton, Mary Trump in first-week sales MORE, will not testify before Congress on Monday, despite a congressional subpoena, according to a letter from his attorney.

House Democrats have threatened Kupperman with contempt for refusing to comply.

ADVERTISEMENT

"We want to assure your clients, again, that it is not Dr. Kupperman who contests your clients' constitutional claim," the attorney, Charles J. Cooper, wrote to the legal counsel for the House committee.

"It is President Trump, and every President before him for at least the last half century, who have asserted testimonial immunity for their closest confidential advisors. If your clients' position on the merits of this issue are correct, it will prevail in court, and Dr. Kupperman, I assure you again, will comply with the Court's judgment."

Read the full letter below.