U.S. President Donald Trump is confronting the extraordinary possibility of being compelled to testify via grand jury subpoena. And any way you slice it, his options don't look pretty, constitutional and legal scholars say.

If Trump challenges the order in the U.S. Supreme Court, he probably loses. If he agrees to testify, he risks committing perjury, resulting in impeachment. If he refuses to comply, he opens himself to political harm and embarrassment — and even jail time if he's held in contempt.

As first reported by the Washington Post, Special Counsel Robert Mueller, who is overseeing the investigation of Russian meddling in the 2016 election, floated the idea of serving Trump with a subpoena to compel him to testify back in March.

The development may have already claimed its latest casualty on Trump's legal team. On Wednesday, White House lawyer Ty Cobb quit. He will be replaced by Emmet Flood, a Republican attorney who defended former president Bill Clinton during his impeachment. That suggests Trump is girding himself for a possible impeachment scenario.

He probably should, given the legal peril that a subpoena from Mueller would put him in, experts tell CBC News.

Seven experts weighed in on what could happen if Trump is indeed subpoenaed by the special counsel. They are:

Jonathan Turley, constitutional law professor at George Washington University

Louis Seidman, constitutional law professor at Georgetown University

Ryan Goodman, former special counsel to the general counsel of the U.S. Department of Defense

​David Sklansky, criminal law professor, Stanford Law School

Mark Osler, law professor, University of St. Thomas

Paul Rosenzweig, senior counsel on independent counsel Ken Starr's investigation of Bill Clinton

Susan Bloch, constitutional law professor, Georgetown University

Can the president be compelled via subpoena to testify?

JONATHAN TURLEY: The president would be on shaky legal ground to challenge a subpoena from Mueller. The federal courts have given presidents a great deal of leeway in terms of the scope and timing of interviews. However, the courts have generally ruled against presidents who claimed a form of absolute immunity.

LOUIS SEIDMAN: It can be done. You serve a subpoena on the White House and you've subpoenaed him, but he would then resist it in court.

RYAN GOODMAN: Mueller has a greater weight of legal authority on his side. He even has a strong basis to get what he wants with a subpoena for any matters that preceded Trump's coming into office.

DAVID SKLANSKY: The reason to think a president needs to comply with a grand jury subpoena is that, first of all, we have a general principle that no person is above the law, including the president.

U.S. President Bill Clinton was impeached on perjury and obstruction of justice in 1998 in a House vote, though he was able to stay in office because a Senate trial never convicted him. (Win McNamee/Associated Press)

Is there legal precedent for this involving U.S. presidents?

GOODMAN: U.S. versus Nixon in 1974 required Nixon to turn over audio tapes under a subpoena from a special prosecutor. That's closely analogous, but not perfect. There was another case involving president Clinton standing trial in a civil suit with Paula Jones, who sued Clinton for alleged sexual harassment.

MARK OSLER: Neither case is a perfect match. The one to subpoena Richard Nixon in 1974, that involved tapes. With the more recent one was with Paula Jones versus Clinton, the distinction there is it's a civil case, not a criminal case. Certainly, the language of the Nixon opinion would seem to cover the situation the court talks about, the overriding need to get evidence in a criminal case, and that overrides the president's interest in privacy.

Media reports say Robert Mueller has compiled a list of more than 40 questions to ask the president as part of his probe into Russia's election meddling. They range from Trump's motivations for firing FBI Director James Comey a year ago to contacts Trump's campaign had with Russians. (Andrew Harnik/Associated Press)

What's so risky about being compelled to testify?

PAUL ROSENZWEIG: The big risk, if you pull back far enough, is impeachment. And impeachment would follow from perjury, from pre-presidential crimes or post-presidential crimes.

TURLEY: Not only would the president have to overcome existing precedent — he would lose protections that he currently enjoys. I'm talking practical protections, not legal ones. Mueller is currently willing to limit questions to just four categories. And he's also willing to limit the time of the questioning. Most importantly, if he's forced into a grand jury, the president would not be able to take his full legal counsel into the grand jury with him, which is a very significant risk.

OSLER: In a grand jury, the witness goes in there and sits alone. There's no judge to moderate the discussion, there's no counsel there. And if you say something untrue in front of a grand jury, that's perjury.

What if Trump refuses to comply with a Supreme Court order?

SUSAN BLOCH: He'd be found in some kind of contempt. If the court ultimately ordered him to comply, and then he refuses, I think that would be impeachable. I really think so. You have to ask yourself, is that a high crime and misdemeanour? Refusing an order from the Supreme Court? And I would argue that it is.

GOODMAN: That would invite a constitutional mini-crisis. It would be a dramatic confrontation between the courts — with the prosecutors on one side, the president on the other — and he could be held in contempt of court. It would more likely result in some kind of voluntary information or the president saying he would plead the Fifth.

Why doesn't Trump just plead the Fifth?

OSLER: He could. But the problem is if he pleads the Fifth in front of a grand jury, the next thing that happens is the prosecutor grants the person "limited-use immunity," which means that they're guaranteed what they say won't be used directly against them, other than perjury. If they still refuse, having been granted limited use immunity, they can be held in contempt of court and locked up. There, we face the prospect of the president of the United States cooling his heels in jail — and that would certainly be unprecedented.

BLOCH: He could take the Fifth, which means "I refuse to testify on the grounds it might incriminate me," but it has political consequences. He already said, why would anyone take the Fifth unless you committed a crime? That would be quoted right back to him.

SKLANSKY: I think refusing to comply with a lawful court order, there's a very strong argument that that's an impeachable offence, and offhand I can't think of a good argument that it's not.

On what grounds might Trump's team challenge a subpoena?

OSLER: The argument would be that the constitution directs criminal charges against the president to impeachment rather than the courts. And because of that, there really shouldn't be a subpoena to the president, if he's the target of the investigation. Because the process should go through Congress.

SEIDMAN: They would argue it violates the president's Article II powers [under the U.S. Constitution], which grant him the executive power. The court has said one aspect of that is executive privilege, which is their right to withhold certain information in certain circumstances, but not when it's needed as evidence in a criminal trial.

BLOCH: They would try and argue along the lines of, "We don't believe you can indict a sitting president." Also, the president has a lot of responsibilities, and these things should have to wait until he's out of office.

SKLANSKY: The argument that a president can't be required to answer subpoenas because it's too much of a distraction is probably less persuasive with this president. It's not a credible claim. We've never had a president who spends this much time watching television.

BLOCH: That's the argument Clinton made in the Paula Jones case — he lost that, but I think he was right then. I think it's a strong argument. I don't think the president should have to deal with all these things while in office.

​TURLEY: He would renew the claim of immunity, which the courts previously rejected. But there's no indication the courts would seriously consider changing the current case law in this area.

SKLANSKY: They might claim that the independent counsel is unconstitutional or has exceeded his authority. I think those are very hard claims to make. But if you're grasping at straws, that's a straw.

But isn't it true that a sitting president can't be indicted?

ROSENZWEIG: I think the Department of Justice policy is pretty clear that a president cannot be indicted, and Mueller is stuck with that policy. The DOJ policy is rightly or wrongly, a binding policy for Mueller, and he works for the DOJ. If your boss has rules, you follow the rules, even if they're stupid rules.

SEIDMAN: In the end, if Trump really insists on having it his way, at some point the law kind of breaks down and now you just have raw power. At that point, you have a real constitutional crisis.

BLOCH: If he were to get impeached or somehow leave office before his term has ended, remember that Vice President Mike Pence would become president. And Pence could pardon Trump. But even if he's pardoned, the pardon is only good for federal crimes such as obstruction of justice. He would still be vulnerable to any state crimes.

ROSENZWEIG: The question of indictment really turns on the answers. There is no — except for contempt — exception to the prohibition. Which is one of the reasons the Justice Department opinion is wrong, in my opinion. If the president murdered his wife today, could he not be indicted until after he left office? That just don't seem right.

Should the president drag out this process with a Supreme Court challenge?​

TURLEY: The matter could be expedited to the Supreme Court, which could hear the issue and rule within a matter of months, but certainly it could push it past the summer. That timing could not be worse for the president. He would be creating this confrontation at the very time the Republicans are facing the critical mid-term elections, so he would guarantee that this issue would define the mid-term elections.

U.S. Vice President Mike Pence would become president if Trump is impeached or leaves office before the end of his term. Pence would have the power to pardon Trump. (Andres Stapff/Reuters)

What about the Justice Department's role in all this?

GOODMAN: Under the Special Counsel regulations, the Department Attorney General, Rod Rosenstein, must be notified of any "significant development," which would include subpoena of a president. And he can overturn a decision by a Special Counsel.

The Justice Department's Office of Legal Counsel appears to have reached a conclusion already in previous opinions that a sitting president can be subpoenaed to testify in a criminal case. The only question boils down to what specific information would be subject to that subpoena.

President Donald Trump this week announced he has hired Emmet Flood, a veteran attorney who represented Clinton during his impeachment process. (Marissa Rauch/Williams & Connolly LLP/Associated Press)

Can Congress compel Trump to testify?

That's a whole other, complicated matter. Turley told CBC's Mark Gollom that "nothing compels him to go."