The US Capitol  is seen early in the morning on October 17, 2019 in Washington, DC.
CNN  — 

Intrigue continues to swirl around President Donald Trump’s former deputy national security adviser Charles Kupperman, a key witness in the ongoing impeachment inquiry who filed a lawsuit Friday asking a federal judge to rule on whether he is obliged to testify before House investigators.

Kupperman, a longtime associate of former national security adviser John Bolton, served in several positions in the Reagan administration and subsequently served as a space operations executive at Lockheed Martin and in Boeing’s missile defense sector.

He is scheduled to testify behind closed doors at 9:30 a.m. ET Monday as part of the House-lead impeachment inquiry. Kupperman’s attorney wrote in a letter Sunday night to House investigators that his client would not appear before the committee until a court has weighed in on the matter, making his deposition Monday morning highly unlikely. House Democrats have threatened to hold Kupperman in contempt if he does not appear for his scheduled hearing – though it could be rescheduled.

Kupperman was tapped to be Bolton’s right-hand man in January.

“Charlie Kupperman has been an advisor to me for more than thirty years, including during my tenure as National Security Advisor to President Trump,” Bolton said in a statement at the time of his hiring. “Charlie’s extensive expertise in defense, arms control and aerospace will help further President Trump’s national security agenda.”

A short stint

But Bolton’s move to add another like-minded GOP hawk to Trump’s national security team raised immediate questions as Trump had already begun indulging his non-interventionist instincts in the Middle East with his plans to pull out of Syria and withdraw some troops from Afghanistan – in spite of Bolton and other aides’ opposition.

Ultimately, the writing was on the wall for Kupperman as he only served in the Trump administration for roughly nine months before departing in September, shortly after Bolton was unceremoniously ousted by the President.

Trump fired Bolton as his national security adviser last month, after Trump made clear he “strongly disagreed” with many of Bolton’s suggestions. Bolton, however, insisted that he had resigned.

Trump had grown increasingly irritated with Bolton for his statements on Iran, Venezuela and Afghanistan, and felt that Bolton could no longer advocate for the President’s agenda, a senior administration official told CNN at the time.

After his departure from the administration, Bolton spoke out against Trump’s policies and secured a book deal, Axios reported.

Kupperman was named interim national security adviser immediately following Bolton’s departure, but sources told CNN that he was never seriously considered for the full time role due to Trump’s belief that he aligned too closely with the views of the man he was looking to replace.

Listening in on the Zelensky call

Despite his relatively short tenure, Kupperman has drawn the interest of House investigators who believe he has first-hand knowledge of Trump’s decisions regarding Ukraine that have come under scrutiny as part of the impeachment probe.

Specifically, Kupperman was listening in on the July 25 phone call when, according to a White House transcript, Trump pressed Ukrainian President Volodymyr Zelensky to investigate former Vice President Joe Biden.

The White House sent a letter to Kupperman directing him not to comply with a subpoena he received from the House committees and maintaining that he would be protected by “constitutional immunity.”

Some Democrats had hoped Kupperman might still appear for his scheduled hearing on Monday, joining other witnesses who have defied White House orders.

Will Kupperman show up?

However, those expectations have been tempered by the filing of Friday’s lawsuit which will almost certainly delay the hearing and could allow Kupperman to avoid testifying all together.

Chuck Cooper, Kupperman’s attorney, maintains in the letter sent Sunday that the lawsuit aims to have the judiciary, not Kupperman, resolve a conflict between the President and House Democrats.

House Intelligence Chairman Adam Schiff, Foreign Affairs Chairman Eliot Engel and acting Oversight Chairwoman Carolyn Maloney sent Kupperman a letter Saturday charging that his lawsuit was “lacking in legal merit” and coordinated with the White House, calling the effort “an obvious an desperate tactic by the President to delay and obstruct the lawful constitutional functions of Congress and conceal evidence about his conduct from the impeachment inquiry.”

“Notwithstanding this attempted obstruction, the duly authorized subpoena remains in full force and Dr. Kupperman remains legally obligated to appear for the deposition on Monday,” the chairs wrote. “The deposition will begin on time and, should your client defy the subpoena, his absence will constitute evidence that may be used against him in a contempt proceeding.”

The lawsuit is raising new questions about Kupperman’s willingness to share information with the House committees and presents a new challenge to Democrats seeking cooperation from close aides of Trump, including Bolton, who is also represented by Cooper.

Cooper declined comment when asked if the lawsuit filed on Kupperman’s behalf is an indication of how Bolton will handle the House request for testimony, but it appears unlikely that he would seek different accommodations for his two clients.

Bolton’s role

CNN reported last week that Bolton’s lawyers have had talks with the three House committees leading the impeachment inquiry about a possible deposition.

As House members who serve on the Intelligence, Oversight and Foreign Affairs committees have been interviewing witnesses in private, some of the committees’ Democrats have said they believe there’s a need for Bolton to testify.

While it remains to be seen if Kupperman or Bolton ultimately end up testifying, their actions are widely viewed as intertwined, with one source telling CNN that the two men are “simpatico.”

They are considered part of the same fringe group of ideologues that has remained relevant in American politics since the Cold War era and, at one time, maintained a growing level of influence in the Trump White House, sources said.

That group includes Frank Gaffney, a controversial figure in Washington who founded the Center for Security Policy think tank where Kupperman previously worked.

All three men share an ideology that is predicated on a deep distrust of diplomatic resolutions and arms treaties as deterrence mechanisms – views that outweigh loyalty to any single administration or president.

This story has been updated with additional developments Monday.

CORRECTION: This story has been updated to reflect that Kupperman and Gaffney are affiliated with the Center for Security Policy

CNN’s Suzanne Malveaux, Jake Lubbehusen, Kylie Atwood and Jeremy Herb contributed to this report.