Cooper also represents Charles Kupperman, Bolton’s former deputy, who had asked a federal court whether he was required to comply with a subpoena seeking his testimony. The White House stepped in to block Kupperman from testifying, and Democrats believed they would have done the same for Bolton. The House withdrew the subpoena for Kupperman this week.

The leaders of the three House committees leading the impeachment inquiry decided on Thursday that they would not issue a subpoena to Bolton, effectively giving up on securing his testimony. Bolton was due to appear for a deposition Thursday, and Cooper informed the committees that his client would challenge a subpoena in court.

A House Intelligence Committee official called Bolton’s posture a delay tactic, and said investigators had no interest in entertaining a months-long court battle. Instead, the official said, Bolton’s refusal to testify would be used as evidence against President Donald Trump of obstruction of Congress.

Several current and former White House officials who have already testified before impeachment investigators have described Bolton as a key witness to the events surrounding Trump’s efforts to pressure Ukraine’s leaders to investigate his political rivals. He at times described Trump’s personal lawyer Rudy Giuliani as a “hand grenade,” and said acting White House chief of staff Mick Mulvaney was involved in a “drug deal” involving potentially withholding critical military aid from Ukraine and refusing a White House meeting between the two presidents.

Cooper rejected the House’s assertion that the attempt to resolve the matter in court was a delay tactic to avoid testifying — or that it had been coordinated in any way with the White House.

Rather, he said the House should have joined the case to secure a resolution.

“If the House chooses not to pursue through subpoena the testimony of Dr. Kupperman and Ambassador Bolton, let the record be clear: that is the House’s decision,” Cooper wrote.

Though the House has argued that a previous federal district court ruling found there were limits on “absolute immunity” for White House aides, Cooper noted that the ruling itself described exceptions for senior officials in charge of national security matters, a label that would apply to both Bolton and Kupperman.

In fact, Cooper notes, the House acknowledged national security as a legitimate basis for asserting absolute immunity when arguing that former White House Counsel Don McGahn should not receive it.

“Here, unlike McGahn, information concerning national security and foreign affairs is at the heart of the committees’ impeachment inquiry,” Cooper wrote, “and it is difficult to imagine any question that the committees’ might put to Dr. Kupperman that would not implicate these sensitive areas.”