In a presidential campaign featuring many firsts, one of the most startling came Sunday night when Republican nominee Donald Trump, to scattered cheers from the audience, pledged to have the Democratic nominee investigated criminally, should he prevail in November.

Specifically, in reference to her actions relating to her private email server, he said: “And I’ll tell you what. I didn’t think I’d say this, but I’m going to say it, and I hate to say it. But if I win, I am going to instruct my attorney general to get a special prosecutor to look into your situation, because there has never been so many lies, so much deception. There has never been anything like it, and we’re going to have a special prosecutor.”

When Hillary Clinton replied that it was “awfully good that someone with the temperament of Donald Trump is not in charge of the law in our country,” Trump quickly added, “Because you’d be in jail.”

Trump’s campaign manager Kellyanne Conway tried to downplay Trump’s threat, later saying it was just “a quip.” However, some prominent lawyers and legal scholars took umbrage at the threat and expressed alarm. Harvard Law professor Laurence Tribe told Fortune that even threatening such a thing was “incompatible with the survival of a stable constitutional republic,” while carrying out such a threat would constitute an “impeachable offense.”

Eric Holder, former U.S. Attorney General in the Obama Administration, immediately tweeted that Trump’s comment rendered him “unfit” for office.

Fortune reached out to all the former U.S. attorneys general that we could locate (including Holder), as well as several other prominent legal authorities and presidential historians, to get their views. Was what candidate Trump proposed legal? Was there precedent for it? Was it good policy? Here are the answers we’ve received so far.

Laurence Tribe, Professor of Constitutional Law, Harvard Law School

(Via email:) “Under the laws and Justice Department regulations governing federal prosecution, a President Trump would not have legal authority to direct the Attorney General to appoint a special prosecutor to ‘look into’ Hillary Clinton’s email situation or the Clinton Foundation or anything else. That’s not within a President’s power.

The only precedents for the kind of vow Trump made in last night’s debate are to be found in dictatorships and banana republics, not the United States. The closest parallel may be what [Viktor] Yanukovych (a former Paul Manafort client) did to [Yulia] Tymoshenko in Ukraine.

Making threats or vows to use a nation’s criminal justice system against one’s vanquished political opponent is worse than terrible policy: it’s incompatible with the survival of a stable constitutional republic and, under our Constitution, would represent an abuse of power so grave that it would be an impeachable offense—one reminiscent of Richard Nixon’s deliberate use of the IRS to go after his political enemies.”

[In a second email, Tribe added that] “some of the political leaders who’ve jailed their political opponents [in the past] have been Hugo Chávez, Recep Erdoğan, Robert Mugabe, Manuel Noriega, Augusto Pinochet and, of course, Vladimir Putin.”

Judge Michael Mukasey (former U.S. Attorney General, 2007-2009, under George W. Bush; U.S. District Judge, 1987-2006, appointed by Ronald Reagan; partner, Debevoise & Plimpton)

Asked in a telephone interview whether Trump could order that a special prosecutor be appointed, “No, that’s not within his powers. I think he said, ‘I would direct my attorney general’ to exercise the power to appoint a special counsel. “The AG’s proper answer would then be, ‘I’ll think about it.’ Or ‘I’ll review the facts,'” Mukasey said. [His point is that the attorney general must exercise independent discretion—not merely carry out the President’s whims.]

Were there any precedents for such an action? “Not as far as I know. The only thing close, ironically, is Eric Holder having said during the 2008 campaign, speaking of [C.I.A. officers’ treatment of terrorism detainees], ‘There has to be a reckoning.” Holder, Mukasey said, though a private citizen at the time, was obviously a likely nominee to head the Justice Department in the event then-candidate Barack Obama won, as he did.

When Holder became A.G., he did, in fact, open an inquiry into the conduct of several C.I.A. officers, according to Mukasey, although the Justice Department had already opened and closed investigations into them. “Which I thought was stunning,” he adds. The new inquiries were ultimately closed without prosecutions also, however. [Holder did not respond to Fortune‘s inquiry seeking comment.]

Would it be wise policy for Trump to reopen an inquiry into Clinton’s emails? “I would think you wouldn’t want to create the impression that this is a banana republic,” Mukasey said. “But that’s not to say it’s inappropriate to reinvestigate. There are lots of serious questions about it.”

“An easy way to moot all this,” he added, “is if Trump were elected, then President Obama could simply pardon her.”

Jim Jacobs, professor at New York University School of Law, specializing in criminal law and procedure

(In a phone interview he said:) “That’s very contrary to the way the Justice Department operates. It is essential that the Department be apolitical with respect to its choice of law enforcement targets and to its exercise of prosecutorial discretion. And very improper if the president were to be making phone calls to the attorney general with respect to a particular target of investigation. I don’t know of any president who has done that. If it had been revealed to have happened in the past it would have been a scandal.

The same with respect to U.S. Attorneys [the top federal prosecutors around the country]. They wouldn’t expect to be called by the White House, and be told by the president, ‘I want you to investigate someone.’ I think that would be grounds for resignation. That would call into question the independence of the law enforcement wing, and put a grave cloud over it.”

Ted Olson, former Solicitor General of the United States, under George W. Bush; partner, Gibson Dunn & Crutcher

(Per Email:) “Justice Department regulations allow the Attorney General the discretion to appoint an independent counsel/prosecutor. . . . I don’t recall such a pre-election commitment, but there well could have been. I’ll stay out of the policy debate.”

Alan Dershowitz, Harvard Law School professor, emeritus, of constitutional and criminal law

“The president is not supposed to influence prosecutorial policies by the Justice Department, which should be independent. It is bad policy to mix politics with law enforcement.”

Dick Thornburgh, former U.S. Attorney General from 1988-1991 under Ronald Reagan and George H.W. Bush; partner at K&L Gates

“The attorney general would clearly have the power to proceed. Whether it would be wise or not is another matter. . . .

The president can instruct the attorney general to do anything, as we learned from Watergate [referring to the instance in which President Richard Nixon ordered attorney general Elliott Richardson to fire special prosecutor Archibald Cox, and Richardson resigned rather than do so]. . . .

There’s very little precedent for a winning candidate for presidency to seek some sort of criminal prosecution as retribution against a recent opponent. I’ve racked my brain. . . .

As a matter of policy, an attorney general might refuse to carry out such a prosecution. . . .

One troubling aspect of this is that one AG has already ostensibly reviewed the record and found no basis for bringing a prosecution. But that doesn’t prevent a successor from reviewing it.”

(We will be updating this post as additional comments arrive. If you have expertise in this area, please write and we will consider including your comments as well.)