An attorney for former 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 and former deputy national security adviser Charles Kupperman said Friday he is "dismayed" that his clients will not be compelled to testify in the ongoing House impeachment inquiry after lawmakers refused to issue subpoenas.

Charles J. Cooper, whose clients declined to testify voluntarily, wrote in a letter to House general counsel Douglas Letter obtained by The Hill that he had hoped to determine in court whether House subpoenas to testify or the White House's assertion of executive privilege took precedence.

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"We are dismayed that the Committees have chosen not to join us in seeking resolution from the Judicial Branch of this momentous Constitutional question as expeditiously as possible," Cooper wrote. "It is important both to Dr. Kupperman and to Ambassador Bolton to get a definitive judgment from the Judicial Branch determining their Constitutional duty in the face of conflicting demands of the Legislative and Executive Branches."

"Dr. Kupperman stands ready, as does Ambassador Bolton, to testify if the Judiciary resolves the conflict in favor of the Legislative Branch’s position respecting such testimony," he added.

House Democrats had scheduled Bolton to appear on Thursday, but said they wouldn't issue him a subpoena after he declined to appear.

"Mr. Bolton would take us to court if we subpoenaed him," a House Intelligence Committee official said in a statement.

"We would welcome John Bolton’s deposition and he did not appear as he was requested today," the official said. "We regret Mr. Bolton’s decision not to appear voluntarily, but we have no interest in allowing the Administration to play rope-a-dope with us in the courts for months."

House Democrats also withdrew their subpoena for Kupperman's testimony on Wednesday.

"Given the schedule of our impeachment hearings, a court process that leads to the dismissal of Dr. Kupperman’s flawed lawsuit would only result in delay, so we have withdrawn his subpoena," an Intelligence panel official said.

Cooper pushed back on this narrative in his Friday letter to Letter.

"The House Chairs are mistaken to say Dr. Kupperman’s lawsuit is intended 'to delay or otherwise obstruct the Committees’ vital investigatory work,'" he wrote.

"Nor has the lawsuit been coordinated in any way with the White House, any more than it has been coordinated with the House of Representatives. 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 added.

The House Intelligence, Oversight and Reform, and Foreign Affairs committees are investigating 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's dealings with Ukraine. Bolton, who had an ugly exit from the administration in September after multiple rifts with Trump, is seen as a particularly important potential witness in the probe.

The former adviser is said to have expressed concern about attempts by Trump and his lawyer Rudy Giuliani Rudy GiulianiThe Hill's Campaign Report: GOP set to ask SCOTUS to limit mail-in voting CIA found Putin 'probably directing' campaign against Biden: report Democrats fear Russia interference could spoil bid to retake Senate MORE to convince Ukrainian President Volodymyr Zelensky to look into Democratic presidential candidate Joe Biden Joe BidenFormer Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick Bloomberg rolls out M ad buy to boost Biden in Florida MORE, a former vice president, after the administration placed a hold on hundreds of millions of dollars in military aid to the country.

—Olivia Beavers contributed.