Not long ago, the idea of removing Rod Rosenstein, the U.S. Justice Department official overseeing the special counsel's investigation into President Donald Trump and Russian collusion, seemed improbable.

And yet doubts about the deputy attorney general's future are taking shape this week as he prepares for a one-on-one with Trump at the White House on Thursday. Some reports are already anticipating Rosenstein's imminent departure.

His ouster, if it comes to pass, could provoke a constitutional crisis.

The prospect of his removal, depending on whether it happens via a voluntary resignation or a firing, could provide a window for the president to install a Trump loyalist capable of derailing special counsel Robert Mueller's investigation.

Here are some of the key questions about Rosenstein's future and the fate of Mueller's probe.

What's this Thursday meeting about?

The pretext is a bombshell New York Times report that Rosenstein last year raised the idea of secretly taping Trump to build a case for his removal under the 25th Amendment of the U.S. Constitution. The amendment deals with removing a president who is deemed unfit to execute the duties of the Oval Office.

Trump allies have long been furious with Rosenstein for appointing Mueller, and the Times article could give Trump reason to fire him, despite the president's oft-repeated attacks on the newspaper's coverage as "fake news."

Trump, shown here speaking at the UN in New York with U.S. Ambassador Nikki Haley, was non-committal about Rosenstein's future in an interview that aired on Monday. (Carlos Barria/Reuters)

"We'll be meeting at the White House, and we'll be determining what's going on," Trump said Monday. White House Press Secretary Sarah Sanders said the president "wants answers" from Rosenstein following the Times story.

Reports have suggested Rosenstein will refuse to resign and will instead demand to be fired if the president wants him out. Rosenstein has refuted the Times report and denies he ever "advocated for" wiretapping Trump or planning for his removal.

Is Mueller's investigation in jeopardy?

It depends on the lawyer who's next in the line of succession, should Rosenstein leave.

Remember that after Attorney General Jeff Sessions recused himself from the Russia investigation, Rosenstein stepped in and tapped Mueller to lead the probe. In his capacity as an acting attorney general for the Russia probe, Rosenstein has been supervising Mueller's work and signing off on his actions, effectively acting as Mueller's boss and chief protector.

In theory, Mueller's work should go on undisturbed, regardless of who has oversight over his team, said Harry Sandick, a former federal prosecutor with the Southern District of New York.

"But it's not clear that a new supervisor will have the same orientation as a career Department of Justice prosecutor that Rosenstein has," Sandick wrote in an email. "And this could influence Mueller's work by limiting his line of investigation or rejecting proposed charges."

If Rosenstein is fired, it would send "shock waves" through the Federal Bureau of Investigation, the Department of Justice and across Capitol Hill, said Greg Brower, former head for the FBI's Office of Congressional Affairs.

"It would be seen by most people … as another attack on the DOJ with a political motivation to undermine the Mueller investigation."

How else could Mueller's work be impeded?

While Brower sees no reason to believe Rosenstein isn't acting with integrity to uphold the rule of law, he notes that he is "certainly in a position, if he wanted to, to interfere with and effectively end the special counsel's investigation."

His potential replacement would have the same powers to undercut Mueller or slow down his work, said Brower, who previously worked with Rosenstein at the Justice Department.

A replacement seeking to shield the president from the investigation might pull Mueller's resources, for example.

Former federal prosecutor Renato Mariotti suggested it might still be possible for Rosenstein to keep his job as deputy attorney general. The scheduled meeting could be a distraction tactic, he said, to take attention away from Supreme Court nominee Brett Kavanaugh's high-profile Senate hearing set for the same day. (Kevin Tanaka, Sun Times/Associated Press)

"Mueller is using FBI resources and, to some extent, DOJ resources," Brower said. "So if all of a sudden he deprived Mueller of half the agents currently on his team, that would hamper his investigation."

As former federal prosecutor Renato Mariotti pointed out, the deputy attorney general also sets the scope of Mueller's authority and "could potentially waste Mueller's time by forcing him to constantly justify every decision he makes."

At the very least, Mariotti said, this might "create a public perception that Mueller is doing something improper."

Denying Mueller's requests to advance his investigation would require congressional notification.

Does it matter whether Rosenstein is fired or quits?

That's an important distinction.

Under the 1998 Federal Vacancies Reform Act, the president has the authority to appoint an acting official already confirmed by the Senate to replace an official who "dies, resigns, or is otherwise unable to perform the functions and duties of the office."

Note that "fired" is not included in the language.

"So it's very clear, if Rosenstein resigns voluntarily, Trump could immediately appoint an acting deputy attorney general who would step into Rosenstein's shoes," and thus assume control of the Mueller probe, Mariotti said.

Things get trickier if Rosenstein forces Trump to fire him. In that case, it would be unclear whether Trump could simply appoint a Senate-confirmed stand-in of his own choosing — but he could expect legal challenges if he attempts to do so.

Critics of the Federal Vacancies Act object to the idea that a replacement officer can be named to an agency without being confirmed by the Senate to work in that agency.

Who would replace Rosenstein?

If he's fired, there's a stronger case for Rosenstein's replacement to come from a predetermined order of succession.

Next in line would be solicitor general Noel Francisco, though some legal experts believe Francisco may also have to recuse himself from overseeing the Mueller probe and cite a conflict of interest, due to his work within the last two years with Jones Day, a law firm that represented Trump.

If he recuses himself, the Mueller investigation's oversight responsibilities would fall to Steven Engel, currently the associate attorney general in charge of the Office of Legal Counsel.

Note that, while nominee ethics agreements mention only a one-year recusal from former employers under the Standards of Conduct, Executive Order 13770, section 1, paragraph 6, expands that recusal obligation to 2 years. See the (not at all random) example below: <a href="https://t.co/equkJdYq7k">pic.twitter.com/equkJdYq7k</a> —@waltshaub

Could a new deputy AG even fire Mueller?

Yes. He or she would have the authority to do so under federal regulations in Section 600, stating the general powers of a U.S. special counsel for specific misconduct, such as a dereliction of duty or a conflict of interest.

Trump allies have tried to portray Mueller, a registered Republican, as beholden to Democratic interests and thus in conflict of interest.

"That's also why you hear Trump talking about how Mueller has a conflict of interest because of a spat over his golf club," said Watergate historian and legal ethics expert Jim Robenalt. "But that's very minor, it's no reason to remove somebody."

Solicitor General Noel Francisco departs the U.S. Supreme Court in this 2016 file photo. Francisco is the No. 3 ranking law enforcement official in the Justice Department. (Jonathan Ernst/Reuters)

Whoever ends up in a position to fire Mueller might be in an unenviable role, noted Sandick.

He was reminded of how former solicitor general Robert Bork, who fired Archibald Cox as the Watergate special prosecutor, had to carry on that legacy.

"Whoever fires Mueller will have that as the top line on his biography for the rest of his life."

Is it politically risky to fire Rosenstein?

Some of Trump's advisers think so. Fox News presenter Sean Hannity, known to speak with Trump frequently, warned the president not to fire "anybody" at least until after the November midterm elections.

Conservatives have argued that doing so might fuel Democrats and independents in battleground states to show up in strong numbers, helping the Democrats win both chambers of Congress, especially if a move against Rosenstein provokes a constitutional crisis.

"Trump's lawyers might view it as potential evidence of obstruction [of justice]," said Brower, and counsel against such action.

Putting aside the timing and possible obstruction accusations, a case could be made for an extraordinary abuse of power by the president.

Might this all just be one big distraction?

Maybe. It's possible the status quo will prevail and Rosenstein will keep his job at the end of the day.

That Thursday's meeting just happens to fall on the same day as embattled Supreme Court nominee Brett Kavanaugh's Senate hearing, when he's expected to be grilled on allegations of sexual assault and misconduct dating back decades. One of his accusers, Christine Blasey Ford, is also scheduled to testify the same day.

"Distraction is a strategy Trump has employed in the past," Mariotti said. "Perhaps I'm cynical, but I think what will likely happen [with Rosenstein's job] is nothing."

He added: "It's hard to imagine something happening on Thursday that would be more important than the hearings for Justice Kavanaugh. But if Rosenstein were fired or resigned, that very well might be it."

Would firing Mueller end the investigation?

That train has already left the station, said Robenalt. Mueller's dossier is getting thicker, and his work continues — even if he doesn't.

Rosenstein has long been criticized by many Republicans since appointing Robert Mueller in May 2017 to investigate links between Russia and the Trump campaign. (Evan Vucci/Associated Press)

"He's gathered a lot of information about money laundering, about how the Trump Organization acted, about knowledge of collusion with Russians," Robenalt said. "Once somebody gets into the position of the [deputy] AG and starts looking into this, they're going to conclude there's way too much there to fire [Mueller]."

Even if Mueller believes he's about to be fired, he could still share his information with prosecutors in other districts — as he has already done with the Southern District in New York.

"Just firing him is not going to end the story," Robenalt said. "It's hard to kill this beast."