Washington (CNN) Lt. Col. Alexander Vindman departed Capitol Hill after dark on Tuesday after spending more than 10 hours detailing his concerns at President Donald Trump's handling of foreign policy. The next morning he returned to his desk as Trump's top Ukraine expert.

The swift return to work for one of the impeachment inquiry's central witnesses -- who the President and his allies decried as a "never-Trumper" with ulterior motives , even as he remains on the President's National Security Council staff -- illustrates the predicament facing staffers as the proceedings advance.

The impeachment crisis that's consumed the White House is causing new turbulence at the National Security Council, where officials are wondering whether their efforts are being undermined and worry the President could sour on the entire body, ignoring its expert advice as he fumes about its role in the current crisis.

Already a bumpy workplace under Trump, the National Security Council's career civil servants now find themselves under fresh scrutiny from an already-skeptical President and his inner circle.

Tim Morrison, the council's Russia and Europe director, worked with Vindman in handling the Ukraine transcript and is expected to testify Thursday.

But Morrison is expected to leave his post, a long-planned departure that was nonetheless complicated by the current impeachment drama.

"After more than a year of service at the National Security Council, Mr. Morrison has decided to pursue other opportunities -- and has been considering doing so for some time. We wish him well," a senior administration official said in a statement.

Sources familiar with the situation at the council say there is a sense of anxiety among some staffers as they see their colleagues and State Department officials being called to the Capitol Hill to testify in the ongoing impeachment inquiry that has prompted unfounded personal attacks from Republican lawmakers, some in the West Wing and even the President himself.

Trump's virulent response to Vindman's testimony -- asserting without evidence the National Security Council's top Ukraine expert is a "never-Trumper" with questionable motives -- sent a cold chill through the council's suite of offices in the Eisenhower Executive Office Building adjacent to the West Wing.

Inside the nearly 150-year-old building next to the White House where the National Security Council is housed, the mood has shifted dramatically.

Uncertain role

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Some National Security Council officials -- particularly those not working on top White House priorities like the Middle East or Asia -- increasingly feel undervalued and sidelined in the policy-making process, according to people familiar with the dynamic.

Like Vindman, much of the National Security Council staff is drawn from the Pentagon, State Department or the US intelligence agencies. Many are careerists who served under previous administrations, Democratic and Republican.

A demoralized air hangs over the council, knowing their work is treated with skepticism by the West Wing and often doesn't drive policy decisions in the way a traditional National Security Council would. Often, officials are pressed into service to try and fix problems created in the West Wing on the back end of the decision-making process.

Concerns they could be swept up into the impeachment proceedings have made that more difficult, according to several sources familiar with the situation. One source noted that several National Security Council staffers are worrying about having to hire lawyers when they don't have the resources to.

Since Trump's ill-fated July call with Ukrainian President Volodymyr Zelensky, staffers have watched as the mundane inner-workings of their typically inconspicuous council -- such as how a transcript of a call with a foreign leader is written reviewed and stored -- become public.

"This is the last thing a civil servant wants to get dragged into," said a former Trump administration National Security Council official who remains in contact with current staff. "The folks that are still there are just trying to keep their head down and focus on their work."

In their depositions on Capitol Hill, two council officials -- Vindman and former Russia director Fiona Hill -- both recalled their concerns at apparent attempts to leverage the US relationship with Kiev for political ends. Each said they raised those concerns with their superiors, and ultimately to National Security Council lawyers, a remarkable demonstration of the internal angst among White House professional staff at the actions taken by the President and they people close to him.

Since testifying, Vindman has faced scrutiny within his own ranks. Not seen as a meticulous note taker, his opening statement before lawmakers raised eyebrows among some colleagues who were concerned he would mix up details and cause doubt about what happened.

Still, White House officials acknowledged that Vindman's credibility stems from his proximity to the call. He was the first official who listened in on the conversation to go under oath, though if House Democrats have their way, he likely won't be the last.

Distance from Trump

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Trump's interactions with much of the National Security Council staff have been fleeting, and he has made a point of announcing that some of the most damaging testimony against him comes from people he doesn't know.

"Why are people that I never even heard of testifying about the call," he wrote on Twitter on Tuesday just ahead of Vindman's arrival, in his military uniform, on Capitol Hill.

The list of senior State Department and National Security Council officials who Trump claims are strangers is long. His senior Russia adviser, the architect of US-Ukraine policy, the top diplomat in Kiev: all unknown to the President, he now claims, as they each sit before congressional lawmakers to detail their unease at how he was conducting himself.

The apparent unfamiliarity with his own administration's senior-most policy aides only confirms what those officials are claiming behind closed doors: that Trump ignored the official diplomatic channels they oversaw in favor of unofficial overtures to Ukraine, led by Trump's personal attorney Rudy Giuliani, meant to surface dirt on political opponents.

On Ukraine, Trump often favored the advice of people outside the government instead of the professionals inside the administration. He once mistook Hill, a longtime expert on Russia, for a clerical aide during a briefing.

New leadership

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The impeachment probe also ramped up at a moment when there was already a cloud of uncertainty hanging over the National Security Council as officials adjusted to their new leader, Robert O'Brien, who replaced former national security adviser John Bolton last month.

Officials who were looking to O'Brien for direction on how to handle the impeachment probe haven't found it, they say. O'Brien has not held any staff-wide meetings to offer encouragement or boost dampened morale. And there has been no instruction to keep their heads down and focus on their jobs -- a popular sentiment inside the West Wing amid the special counsel probe.

Despite that, staffers have continued with their work anyway, with some telling colleagues they're eyeing the exits.

Already, O'Brien has moved to slash the size of the National Security Council after the President lashed out against perceived leaks. Earlier this month, O'Brien told White House staff he planned to cut council staff by nearly half and increase the percentage of political appointees in the process, sources told CNN.

That would likely weaken a staff that was already watered down under Bolton, whose aides insisted he was just streamlining bureaucracy.

O'Brien told White House staffers and outside allies that he plans to shrink the National Security Council staff by about 50% by early 2020, reducing total staff numbers through attrition from around 200 people now to a total staff of about 120.

Speaking on Fox Business Network after the meeting, O'Brien said the council had "just ballooned to a massive bureaucracy" during the Obama administration. O'Brien did not address his staff until weeks after being tapped for the job, and several officials said few had a clear understanding of his leadership style or how he might interact with those who he did not have a hand in hiring -- uncertainty that complicated day-to-day work for National Security Council staffers.

Now, those staffers must also deal with additional complexities brought on by the impeachment process, even prior to the subpoenaing of two National Security Council officials: Vindman and Morrison.

Bolton looming

Meanwhile, those National Security Council officials who may be looking to take a cue from their former boss, Bolton, have been given little public indication as to how he plans to approach his own potential involvement in the inquiry. Bolton has gone radio silent since reports that House investigators were interested in hearing from him first emerged, leaving staffers to decide for themselves what to do going forward.

His former deputy, Charles Kupperman -- who retains the same lawyer -- threw a wrench in the impeachment process last week filing a lawsuit asking a judge to decide if he is obligated to appear, a move that appeared to be an effort to delay his testimony.

It remains unlikely that Kupperman's lawyer will seek an alternate approach for Bolton, whose testimony now appears to lay in the hands of a federal judge. Prior to the suit being filed, sources close to Bolton told CNN they would be surprised if he would be willing to appear before the House committees despite his unceremonious firing by Trump.

Bolton is keeping his plans close to the vest, raising questions about how current National Security Council staffers who have maintained a close relationship with him will handle a possible subpoena or request to testify.