Robert Mueller. Alex Wong/Getty Images President Donald Trump has again raised speculation that he could move to fire Robert Mueller, the special counsel overseeing the FBI's investigation into Russia's interference in the 2016 election.

Trump could order Mueller's dismissal. But he would not be able to fire him directly unless he ordered the repeal of the special-counsel regulations adopted in 1999, according to Neal Katyal, the former acting solicitor general, who helped draft the regulations.

But that would be an "extravagant" move, Katyal wrote in a Washington Post column on May 19. If Trump wanted Mueller gone, it is more likely he would order Deputy Attorney General Rod Rosenstein to fire him.

And even that would be shocking.

"Either of those actions was unthinkable to us back in 1999, for we understood that President Richard Nixon's attempt in this regard ultimately led to his downfall," Katyal wrote. He was alluding to the resignations of Attorney General Elliot Richardson and Deputy Attorney General William Ruckelshaus in October 1973 after they refused to follow Nixon's orders to fire the special prosecutor investigating Watergate, Archibald Cox.

"At the same time, after Trump's firing of FBI Director James B. Comey this month, many things once thought beyond the realm of possibility look less so now," Katyal said.

Andrew Wright, a professor of constitutional law at Savannah Law School, said Trump had the power to order Rosenstein to fire Mueller. But he said the special-counsel regulations "require termination only for cause," meaning Trump would have to convince Rosenstein that there are grounds to dismiss Mueller.

Department of Justice regulations stipulate that the attorney general — or, in this case, Rosenstein, as Attorney General Jeff Sessions has recused himself from matters relating to the investigation — "may remove a special counsel for misconduct, dereliction of duty, incapacity, conflict of interest, or for other good cause, including violation of departmental policies," and must "inform the special counsel in writing of the specific reason for his or her removal."

There is no evidence that Mueller has engaged in any inappropriate behavior that would warrant his removal. He was also already granted a waiver by the Justice Department to lead the probe despite a possible conflict of interest stemming from his law firm's representation of some of the people caught up in the investigation, including former Trump campaign manager Paul Manafort and Jared Kushner, the president's son-in-law and senior adviser.

For Trump to order Mueller's firing with no good reason, then, "would be insane even by Trump-era standards," Wright said. And it's possible Rosenstein could refuse, meaning he would have to resign or Trump would have to fire him, which would create a situation eerily reminiscent of Nixon's "Saturday Night Massacre."

Asha Rangappa, an associate dean at Yale Law School and former special agent in the FBI's counterintelligence division, said in an interview that Nixon's firing of his special prosecutor was what "got the ball rolling on his impeachment," indicating Trump's constraints are more political than legal at this point.

If Trump ordered Rosenstein to fire Mueller or tried to do it himself after repealing the special-counsel regulations, Congress would then need to exercise its constitutional power to check the president, Rangappa said.

But even if Congress created an independent commission to continue the Trump-Russia probe — as Democratic Rep. Adam Schiff has said he would pursue if Trump fired Mueller — it does not have the power to press charges or seek civil penalties. And while Mueller's dismissal would not necessarily mean the end of the FBI investigation, the Justice Department ultimately has the power to not bring charges against someone, even if it's sitting on a mountain of evidence.

"This is why the political independence of the DOJ is so paramount," Rangappa said. "The department needs to be able to make the decision to prosecute or not independent of what the president wants."

Finally, it's possible Rosenstein and Trump could work out a deal in which Rosenstein would not fire Mueller but limit his power and the scope of his investigation. Katyal wrote that while Mueller's investigative authority looked fairly broad as it was outlined by the Justice Department last month, he did not have the "sweeping powers" afforded to someone currently in government who had been confirmed by the Senate.

"In 2003, when Patrick Fitzgerald was appointed to investigate leaks that identified former CIA officer Valerie Plame, his appointment letters made clear that he was granted 'all the authority of the attorney general,' which was 'plenary,'" Katyal said. He added that those powers were unavailable to Mueller and "stand as a reminder" that he "operates as a subordinate to the Justice Department, not as Rosenstein's equal."