Washington (CNN) Rod Rosenstein goes to work every day knowing that it could be his last as the deputy attorney general.

Congressional Republicans are threatening to impeach him for failing to turn over an unredacted version of his 2017 memo laying out the scope of the special counsel's investigation. President Donald Trump has called the Department of Justice's handling of the Russia probe -- which Rosenstein supervises -- a "witch hunt" while threatening via tweet that "at some point I will have no choice but to use the powers granted to the Presidency and get involved."

Trump went on to call the FBI raid last month on the offices of his personal attorney Michael Cohen -- which Rosenstein could have stopped but didn't -- a "disgraceful situation." Finally, the President's response to the direct question of whether he was planning to fire Rosenstein didn't provide a lot of job security: "You figure that one out."

Yet people close to Rosenstein say he is staying cool in the midst of the storm.

"He's like shockingly fatalistic," said friend Jim Trusty, an attorney at Ifrah Law in Washington who's a former Justice Department colleague of Rosenstein's. "There's a strange calm to Rod in terms of this fire. I think everyone around him is freaking out, but he just kind of rides through it and says, 'I'm going to keep doing my job as best I can.' "

Rosenstein's own take on his predicament came at a congressional hearing in December, when he was asked by Rep. Hank Johnson, a Georgia Democrat, if he was afraid of Trump firing him. "No, I'm not, congressman," Rosenstein replied after a chuckle.

But there was nothing humorous about his response to his House Republican critics earlier this month: "There have been people who have been making threats privately and publicly against me for quite some time, and I think they should understand by now that the Department of Justice is not going to be extorted."

On the Russia investigation itself, though, Rosenstein has remained silent, and he declined to be interviewed for this article.

"With Rod, if you scratch the surface, then you get more surface. But that's him. He is inscrutable publicly. Professionally, he is devastatingly effective. He is methodical. He is thorough," said Andy White, a close Rosenstein friend and former colleague.

Lifelong Justice Department official

Since graduating from Harvard Law School in 1989, Rosenstein has spent his entire career at the Justice Department: first at department headquarters as a prosecutor in the Public Integrity section and then in various other departments. In 2005 he was appointed US attorney for Maryland by George W. Bush, where he supervised cases ranging from MS-13 gang crimes to terrorism to public corruption.

"He's been aggressive, and he has not shied away from the political spotlight when it comes to prosecutorial decisions and building cases and convicting people that might be very popular and very unpopular," Trusty said.

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Rosenstein was known in the US attorney's office for his skills arguing before a jury and for something more unusual: sending informational memos to his staff before federal holidays.

"We used to always get these emails in the US attorney's office like 'Here's what happened in 1812 and this is why we're taking the day off,' " Trusty said. "He's very academic and very philosophical. He's a historian."

Trump appointed Rosenstein as deputy attorney general in April 2017 and he was confirmed 94-6 by the Senate. Back then, he was a White House favorite.

"He's highly respected, very good guy, very smart guy. The Democrats like him, the Republicans like him," Trump told NBC last May. White House press secretary Sarah Sanders called him "essentially the gold standard at the Department of Justice."

The Comey memo

His reputation with Republicans was buoyed by his now well-known memo -- written after just two weeks on the job -- criticizing then-FBI Director James Comey for his handling of the Hillary Clinton email investigation. The memo called Comey's actions "a textbook example of what federal prosecutors and agents are taught not to do," and Trump fired Comey the same day he received it.

It's unclear just how Rosenstein anticipated his memo being used in the President's justification to remove Comey, especially after Trump decided to publicly release it.

"I think he had to know it was going to be used in some degree. I don't think that he realized that the President was going to put Greyhound bus tracks on his back with that memo," White said. "I don't think that he realized it was going to be used in that way."

For his part, Rosenstein testified before Congress last May that his memo was the result of "long-standing concerns" about Comey's actions, adding, "My memorandum is not a press release. It is a candid internal memorandum about the FBI director's public statements concerning a high-profile criminal investigation. ... I wrote it. I believe it. I stand by it."

But those close to Comey, like former aide Josh Campbell, don't see Rosenstein's intentions as quite so pure, noting that Rosenstein knew of Trump's desire to fire Comey when he wrote the memo. "I think the motive [was] to keep his job," Campbell said.

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Rosenstein's controversial part in the Comey firing would pale in comparison to the role he could be forced to play in another possible firing at the Department of Justice: that of special counsel Robert Mueller. After Attorney General Jeff Sessions recused himself from the Russia investigation, Rosenstein appointed Mueller -- and is now the man standing between Trump and any move to fire the special counsel.

"If [Trump] asks Rod to fire Mr. Mueller, Rod would resign. That's my guess," White said. "Because at that point, it is untenable. You have a President who is not respecting the process and not respecting the Constitution."