READ: Former White House official refuses to testify

A former White House official on Monday told the House Intelligence Committee he would not testify in the congressional impeachment probe into President TrumpDonald John TrumpSchiff pleads to Senate GOP: 'Right matters. And the truth matters.' Anita Hill to Iowa crowd: 'Statute of limitations' for Biden apology is 'up' Sen. Van Hollen releases documents from GAO investigation MORE given the White House's argument of immunity.

Charles Kupperman, the former deputy to former White House national security adviser John BoltonJohn BoltonSenate Republicans confident they'll win fight on witnesses Hakeem Jeffries tells Senate in impeachment proceedings they should subpoena Baseball Hall of Fame after Jeter vote Susan Collins asked Justice Roberts to intervene after Nadler late-night 'cover-up' accusation 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.

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"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.