WASHINGTON — President Trump is taking a beating from within his own administration on impeachment. And it could get worse by week’s end.

On Tuesday, National Security Council member Alexander Vindman told Congressional investigators that he was so alarmed by Trump and a top U.S. diplomat’s actions toward Ukraine that he voiced his concerns to NSC lawyers. Naturally, his remarks triggered a rage-tweetstorm from the president.

But Vindman isn't even this week’s star witness from Trump's NSC. That would be Tim Morrison, an NSC member tasked with overseeing policy for Russia and Europe, who is scheduled to testify on Thursday. According to what’s already been said in closed-door testimony, his testimony could be the most explosive yet.

Given Morrison's role as Trump’s top Russia aide, he’s well-positioned to fill in key details of President Trump’s alleged efforts to push Ukraine to investigate his political rivals — as well as knock down Republicans’ defense that other witnesses’ credible allegations of quid pro quo aren’t valid because they were based on secondhand knowledge.

“He brings this ever closer to Trump’s doorstep,” said Richard Arenberg, a longtime former congressional Democratic staffer who has foreign policy and intelligence expertise.

Morrison was mentioned 15 separate times by Acting U.S. Ambassador to Ukraine Bill Taylor in last week’s damning testimony, and he could corroborate some of Taylor’s most troubling claims.

Morrison’s attorney told VICE News his client plans to come to share what he knows, in defiance of the White House’s blanket ban on administration officials testifying.

“If subpoenaed, Mr. Morrison plans to appear for his deposition,” Morrison’s attorney Barbara Van Gelder said in an email, declining to discuss any part of his planned testimony.

Morrison will be the second NSC adviser to testify this week, after Vindman’s testimony Tuesday. According to Vindman’s prepared opening remarks, he was so “concerned” with Trump’s call with Ukraine’s president that he took his worries to the NSC’s top attorney — and before that confronted a top Trump official for pressuring Ukraine to investigate Trump’s political rivals.

But Morrison may have even more damning information about Trump’s push to force Ukraine to investigate the Bidens.

He’ll be a tough witness for Republicans to ignore.

Morrison has spent his career as a leading Capitol Hill foreign policy hawk. He’s long held a dim view of Russia, which stands to benefit from a damaged U.S.-Ukrainian relationship, and according to those who worked with him, he fiercely advocated for more robust support for Ukraine.

“Tim was an excellent staffer, the best”

He’s also known personally by a number of Republican members: Morrison’s last job before joining the NSC last summer was as the top GOP policy staffer on the House Armed Services Committee. Before that he was then-Sen. Jon Kyl’s (R-Ariz.) top foreign policy adviser, helping Kyl lead the Senate charge against ratifying a nuclear arms-control treaty between the U.S. and Russia.

“Tim was an excellent staffer, the best,” Kyl told VICE News in an email.

Morrison was brought onto the NSC in June by then-National Security Adviser John Bolton, a fellow saber-rattling foreign policy hard-line hawk, to focus on nuclear arms agreements. He moved over to work on Russia and Europe when Trump forced out Fiona Hill, another hawk and Bolton ally, in mid-summer. Bolton was fired on Sept. 10 after reportedly attempting to stand up to Trump and Giuliani on Ukraine, but Morrison has stuck around.

That gives him a unique view into what Trump and his underlings were doing in Ukraine.

Taylor said in his opening statement that Morrison relayed a conversation that U.S. Ambassador to the European Union Gordon Sondland had with a top Ukrainian official.

Taylor also said that Morrison was on Trump’s now-infamous phone call with Ukrainian President Volodymyr Zelensky on July 25. According to Taylor, Morrison said the call “could have been better.” Morrison also allegedly told Taylor that Trump had suggested the Ukrainian president meet with Attorney General Bill Barr as well as Rudy Giuliani, Trump’s personal attorney, who allegedly set up a shadow foreign policy to pressure Ukraine to do Trump’s political bidding.

Those already convinced of Trump’s wrongdoing say Morrison could help clear up any ambiguity for lawmakers.

"He’ll have firsthand information,” former U.S. Ambassador to Russia Michael McFaul told VICE News. “The more first-hand information, the clearer the story. But again, for me the story is very clear."

On August 22, according to Taylor, Morrison told him that Trump “didn’t “want to provide any assistance at all” to Ukraine and that “it remains to be seen” whether the administration still stood by America’s long-held policy of strong support for Ukraine.

A number of Democrats on the trio of committees responsible for the impeachment investigation declined to talk specifics of what they want Morrison to address. But they indicated that he could help them fill in gaps and corroborate other testimony like Taylor’s.

“It's like a bank robbery where you have a dozen witnesses and you want to call in each witness to fill in a different part of the picture. And we are getting a very textured and fine-grained portrait of exactly what happened,” Rep. Jamie Raskin (D-Md.) told VICE News.

Morrison’s testimony could further fill in the picture. But it’s unclear exactly what type of witness he will turn out to be. There are signals that he wasn’t comfortable with what Trump and his aides were doing. But he’s also an aggressive conservative warrior known for his sharp elbows who’s spent his career warring with Democrats — a man the Daily Beast described as a “nuclear superhawk” whose Twitter avatar was a red MAGA-style hat that read “MAKE DETERRENCE GREAT AGAIN.”

Even those who’ve worked with Morrison aren’t sure how things will play out on Thursday.

One former Kyl staffer who worked closely with Morrison said he had “incredibly patriotic convictions” and wouldn’t do anything to harm national security, but that he was also a good foot soldier and “an incredibly loyal person” who “knows when to keep his mouth shut.”

And Morrison’s personal loyalties may lie less with Trump and more with Bolton, who brought him in and reportedly described what Giuliani and his cronies were trying to accomplish in Ukraine as a “drug deal.” Bolton is reportedly in talks with the committee to come and testify himself, a bad sign for Trump.

Another former Morrison colleague from Kyl’s office said if Morrison was alarmed, he wouldn’t shy away from letting the committee know.

“He was squeaky-clean. If this guy’s freaked out, that’s completely legitimate to me,” said the former GOP Hill staffer. “He would do the right thing, too. I would never see him engaging in any kind of cover-up.”

Greg Walters contributed to this story.