(CNN) John Bolton has already left his mark on the impeachment inquiry into President Donald Trump, despite skipping a scheduled deposition Thursday morning.

Trump's former national security adviser is at the center of several key events related to the investigation. Those include suggestions that he had raised concerns about the President and Ukraine, calling efforts by some top officials to help push for investigations into former Vice President Joe Biden and matters related to the 2016 election a "drug deal," according to testimony last month from former top Russia adviser Fiona Hill

Ultimately, the House committees opted not to subpoena Bolton after his attorney threatened to fight such a move in court, according to a committee official, and, unsurprisingly, the former national security adviser was a no-show Thursday.

"We would welcome John Bolton's deposition and he did not appear as he was requested today. His counsel has informed us that unlike three other dedicated public servants who worked for him on the NSC and have complied with lawful subpoenas, Mr. Bolton would take us to court if we subpoenaed him," the official said in a statement provided to CNN.

"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. Rather, the White House instruction that he not appear will add to the evidence of the President's obstruction of Congress," they added.

Several witnesses in the probe have already testified that Bolton had concerns about Trump's dealings with Ukraine and encouraged his staff to sound the alarm about potentially illegal actions by the President's personal lawyer, Rudy Giuliani. But despite those apparent misgivings, it appears Bolton has little interest in playing the role of star witness for House Democrats.

"Bolton still wants to be a player in GOP politics and Trump still has such high approval ratings," a source close to Bolton said.

"So far, he has tried to walk that tightrope. I expect he will continue to do that," the source added, noting that Bolton is unlikely to try to take on Trump directly due to concerns that attacking the President might make it difficult for Bolton to attract wealthy GOP donors to his super PAC.

Bolton has already injected $50,0000 into the campaigns of conservative Republican candidates.

Despite his abrupt and unceremonious departure from the Trump administration, Bolton's willingness to cooperate in the impeachment proceedings remains a mystery.

He has kept a low profile in recent weeks and stayed tight-lipped about his plans regarding a potential deposition -- not even discussing the matter with some of his closest allies. In fact, Bolton has taken multiple trips abroad in recent weeks, including a stint in Asia, just as the pace of impeachment proceedings began to intensify and several of his former staffers from the National Security Council prepared to testify, according to sources familiar with his trip.

Not a 'Never Trumper'

While Trump has labeled some witnesses in the impeachment inquiry -- including another career official who still serves on the National Security Council -- as "Never Trumpers," the President would have a hard time making the same case about Bolton.

A hawkish neoconservative who served in the Republican administrations of Ronald Reagan, George H.W. Bush and George W. Bush, Bolton famously promised Trump he "wouldn't start any wars" when he was hired last March.

His reputation followed him to the White House, where he repeatedly clashed with the President over various foreign policy issues, including Iran and North Korea -- a dynamic that ultimately led to his ouster in September.

Bolton was kicked out of the White House just one day before the hold on the US assistance to Ukraine was lifted. He had been gone for about two weeks when the White House released the transcript of the July 25 call between Trump and Ukrainian President Volodymyr Zelensky.

Bolton was not on the call itself, but his deputy Tim Morrison was listening in.

It is unclear what kind of readout Morrison provided to Bolton, but Morrison told lawmakers that he had seen nothing illegal about the call but was fearful of it leaking.

Charles Kupperman, Bolton's former deputy at the National Security Council, who shares the same legal team, failed to appear under subpoena last week after filing a lawsuit asking a federal judge to determine if he was obligated to testify. Kupperman's subpoena was withdrawn Wednesday as House Democrats moved to avoid delays caused by court proceedings.

While it remains unclear what, if anything, the withdrawal of Kupperman's subpoena means for his former boss, sources close to Bolton have told CNN that even if he does appear Thursday, they do not expect his testimony to be explosive, despite any lingering feelings of animosity he may have toward the President.

Bolton's balancing act may include laying blame on several individuals who have come under scrutiny in the impeachment proceedings, including Giuliani, US Ambassador to the European Union Gordon Sondland and acting White House chief of staff Mick Mulvaney.

"He will say that Giuliani, Mulvaney and Sondland, to a degree, were being disloyal to the President," a source close to Bolton said. "John will say, 'I was the guardrail while Giuliani and Mulvaney were saying let Trump be Trump.' "

Bolton has a book deal in the works, which may also deter him from attacking the President directly.

Role in the impeachment depositions so far

Bolton has featured heavily in testimony provided by other witnesses, including current and former officials who have corroborated allegations that Trump attempted to pressure Ukraine to investigate his domestic political rival with the help of Giuliani and others.

Newly released transcripts of House depositions highlighted how several witnesses knew Trump was pushing for political help from Ukraine this summer. They also knew that the US government was holding up aid from Ukraine while seeking a public announcement that Ukraine would launch investigations into Trump's political rivals, including former Vice President Joe Biden and the Democratic National Committee.

Specifically, Bolton advised members of his National Security Council staff to tip off White House lawyers about potentially illegal activity being carried out by Giuliani, according to sources familiar with last month's testimony by Hill, who is the President's former top Russia adviser.

According to sources familiar with the testimony, Hill quoted Bolton as saying that Trump's lawyer, who was freelancing on Ukraine policy apparently at the President's request, was a "hand grenade" who was "going to blow everybody up."

Hill also said Bolton warned her that he would not get caught up in what he referred to as a "drug deal" being cooked up on Ukraine by Sondland and Mulvaney -- referring to their efforts to secure a commitment from Ukraine to open multiple investigations for domestic political gain.

Bolton's advice to sound the alarm followed a meeting two weeks before the now-notorious July 25 call in which Trump pressed his Ukrainian counterpart to investigate former Vice President Joe Biden and his son, Hunter.

While Bolton appeared to be aware of concerns regarding Giuliani's dealings with Ukraine, it appears he did little to personally stop it.

Former Special Envoy for Ukraine Kurt Volker told the investigating House committees that Bolton "did not engage" on his warnings about Giuliani undercutting established foreign-policy efforts in Ukraine.

According to a transcript of his testimony, Volker had a conversation with Bolton about Giuliani "earlier in August," in which the envoy said, "Basically the same as with Secretary (of State Mike) Pompeo: 'I want you to know Giuliani's out there spinning these narratives. I'm concerned that this is affecting the President's views of Ukraine.' "

It does appear, however, that Bolton was working behind the scenes to help lift the freeze on military aid to Ukraine, according to testimony from the top US diplomat in Ukraine, Bill Taylor.

Taylor testified that he "was told a couple of times by people at State and people at the NSC that the Secretary of Defense, Secretary of State, the National Security Adviser, and the head of the CIA all strongly supported the resumption of this assistance," referring to the Ukraine aid.

Taylor also noted that he "had known from earlier conversations with people that Bolton was working with the two secretaries and the Director of the CIA to get this reversed. So he confirmed that and urged me to make my concerns known to the Secretary again."

In his opening statement, Taylor relayed that Bolton had wanted him to send a cable to Pompeo.

"It was he who suggested then at that meeting that I write this note to Secretary Pompeo, which I did the next day," Taylor said in testimony. "He indicated that he was very sympathetic."

CNN has previously reported Bolton was among those administration officials who opposed the aid freeze from the outset, believing the aid was critical to US national interests and that foreign policy should not be conducted outside the interagency process, something that he had been warned by several officials was occurring, according to their testimonies.

Bolton has long emphasized the importance of process, telling those who worked for him that it can serve as their protector in cases when they have to listen to the person elected.

"Always get the process right: That way your opponents engage you on substance," Bolton would tell staffers, according to those who worked for him.

Bolton will almost certainly be forced to reveal whether he's followed his own advice should he testify sometime in the future.

This story has been updated with additional developments Thursday.