WASHINGTON — White House officials were scrambling Thursday to figure out how to counter the House Democrats' impeachment inquiry, with one source familiar with the situation describing a sense of “total panic” over the past week at the lack of a plan to address the new reality.

There appears to be rising “anxiety, unease, and concern” — as one person close to the White House described the mood in the West Wing — that the whistleblower’s allegations could seriously wound the president and some of those around him. “There’s not a lot of confidence that there’s no there there,” this person said.

White House officials remained unsure of how to proceed, not only because there is no apparent plan to deal with the situation, but because the allegations are so serious that the usual methods the president has used to successfully escape past controversies may not apply: “This doesn’t look like something that’s going to be overtaken by the next news cycle,” the person said.

Another person familiar with the discussions described the mood inside the White House as “shell-shocked,” with increasing wariness that, as this impeachment inquiry drags out, the likelihood increases that the president could respond erratically and become “unmanageable.”

That concern was echoed by another source, who said that some around the president anticipate he will engage in more “impulsive” behavior, with pressure expected to build on him daily during the impeachment inquiry.

That’s sparking worries that Trump could display increasingly unpredictable behavior and lash out in unexpected ways — both a presidential and a political concern in an election year.

With his presidency facing what may be its biggest threat yet, Trump has cycled from offense to defense, reviving a strategy that he viewed as effective during Robert Mueller’s Russia investigation. He tried to downplay his request for the Ukrainian president to help investigate his political rivals, to divert attention to actions by Democrats and presidential contender Joe Biden, and to discredit the whistleblower as having partisan motives.

But while many in the White House are battle-tested from the Mueller investigation, this time is starting to feel different, aides and advisers said.

For a president whose brand is viewed as strength, Wednesday’s press conference made him look defeated, said one person familiar with the situation. While Trump relished questions on the Russia investigation, he seemed to be in no mood to answer questions about Ukraine.

Some allies of the president believe the damage could be mitigated, with some seizing on the fact that the whistleblower did not have first-hand knowledge of the controversial call in question between Trump and Ukrainian President Volodymyr Zelenskiy — a fact that they argue damages the whistleblower's credibility. And, as often happens, when the president is under siege, that fact energizes his core supporters in his defense.

But one Republican strategist close to the White House points out that the president’s base approval ratings are already sky-high — and impeachment will do little to win over new supporters. “There is no upside to having the next four months focused on the president’s impeachment,” particularly if it stymies his legislative agenda and dominates the news cycle.

Another of the sources said that those who were arguing that impeachment could help the president weren’t fully grasping how much of a wild card his response could be.

Worried White House officials have been reaching out to advisers for help in assembling an impeachment response team. With the effort still in the early phases, it remained unclear whether any type of war room-style effort would come to fruition or what form it might take, according to a person briefed on the plans.

But there was an acknowledgement that a coordinated legal and communications strategy was needed, with a clear leader able to marshal a public relations offensive to counter the release of the scathing whistleblower report and the momentum its release has given Democratic impeachment efforts, that person said.

The lack of a long-term strategic vision has been starkly visible, as when Secretary of State Mike Pompeo and Treasury Secretary Steve Mnuchin indicated they were not going to release the memo describing Trump's call with the Ukrainian president — and then 48 hours later feeling that there was no choice but to release the document, according to a source familiar with the situation.

It is anticipated there will be more pressure now than during the investigation by Mueller, who was mostly silent, speaking only through his indictments.

One person who may become involved in developing an impeachment response is Corey Lewandowski, Trump’s outside pit bull and former campaign manager, according to the person involved in the matter. But Lewandowski told NBC News that it was not accurate to suggest he would lead such an effort.

Full coverage: Trump impeachment inquiry

“For the last five years, I’ve done everything I can to support the president and his agenda," he said, "but I have had no discussions with the president and his team at all about joining a team to push back on this fake impeachment narrative.”

Trump's outside lawyer from the Mueller investigation, Jay Sekulow, will be involved in the impeachment efforts. “We will deal with matters as appropriate," Sekulow said when reached by NBC News for comment.

The White House currently lacks a lawyer experienced in impeachment, now that Emmet Flood, who worked on the Clinton impeachment and guided the White House through the end of the Mueller investigation, has left. The current White House counsel, Pat Cipollone, lacks that type of experience.

One source familiar with the White House strategy said that overall, officials were nowhere near the point of bringing people on board or setting up war rooms — but acknowledge they need a coordinated, all hands-on-deck response effort and someone who can lead a political fight, the person said.