House Democrats have signaled they are moving on from their efforts to obtain testimony from 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 after they say a lawyer for Bolton threatened to file a lawsuit if their client was subpoenaed.

Democrats had scheduled for Bolton to testify voluntarily on Thursday as part of the chamber's impeachment inquiry 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's dealings with Ukraine, but sources ahead of his testimony said it was unlikely that the longtime GOP hawk would appear.

A House Intelligence Committee official later said in a statement that a lawyer for Bolton notified the panel that "Mr. Bolton would take us to court if we subpoenaed him."

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"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."

A spokesperson for Bolton did not immediately respond to a request for comment. The Hill has reached out to Bolton's lawyer for comment.

Lawmakers had hoped to get testimony on Thursday from Bolton, who is said to have clashed with the president on a series of key foreign policy matters.

Democrats viewed Bolton as a possible star witness, with Intelligence Committee Chairman Adam Schiff Adam Bennett SchiffSchiff to subpoena top DHS official, alleges whistleblower deposition is being stonewalled Schiff claims DHS is blocking whistleblower's access to records before testimony GOP lawmakers distance themselves from Trump comments on transfer of power MORE (D-Calif.) calling him a “very important” witness late last month.

In particular, Bolton was involved in some of the most explosive events related to Trump's contacts with Kiev that are now at the heart of the House impeachment investigation.

Bolton, who departed the White House last month amid conflicts with Trump over major foreign policy matters, is said to have raised concerns about efforts by the president and his personal attorney Rudy Giuliani to get Ukrainian President Volodymyr Zelensky to investigate 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, one of his top 2020 rivals.

Lt. Col. Alexander Vindman, the National Security Council's top Ukraine expert, testified last month that during a July 10 meeting, U.S. Ambassador to the European Union Gordon Sondland “started to speak about delivering the specific investigations in order to secure the meeting with the President, at which time Ambassador Bolton cut the meeting short,” according to a copy of his opening remarks.

The reported pledge to file a lawsuit in the face of a subpoena for Bolton's testimony would follow the same playbook as Charles Kupperman, who served as a deputy to Bolton before his departure and who asked a judge to decide whether he should appear for testimony.

In a lawsuit for Kupperman, his attorney had previously argued that the former aide was caught in a “classic Catch-22,” between the request to testify and the Trump administration's order seeking to block former and current administration officials from testifying, citing executive privilege.

The former official asked a court to help him decide which branch of government to listen to: the executive branch's immunity claim or the legislative branch's subpoena.

But Democrats on Wednesday withdrew their subpoena for Kupperman's testimony, signaling a desire to maintain the fast-paced nature of their inquiry that is about to enter a public phase with open hearings scheduled for next week.

"There is no proper basis for a witness to sue the Congress in court to oppose a duly authorized congressional subpoena," a House Intelligence Committee official said Wednesday, adding that they do not expect to reissue the subpoena.

"Nevertheless, 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."

Still, both Democrats and Republicans say they wanted to hear from Bolton.

Rep. Mark Meadows Mark Randall MeadowsWhite House chief of staff knocks FBI director over testimony on election fraud Anxious Democrats amp up pressure for vote on COVID-19 aid Pelosi hopeful COVID-19 relief talks resume 'soon' MORE (R-N.C.), a member of the Oversight and Reform Committee and a close Trump ally, suggested Thursday that he'd like to see Bolton testify, characterizing more transparency in the process as "a good thing."



"But," he quickly added, "that's not my call — that's certainly Chairman Schiff's call."



Meadows said he hasn't spoken directly to Bolton, but based on the testimony of other witnesses, he's not worried that Bolton's appearance would do any damage to Trump's case.



"I'm not concerned about any testimony that Ambassador Bolton might provide," Meadows said.

Mike Lillis contributed.