Special counsel Robert Mueller threatened to subpoena President Trump in order to force him to testify in the Russia investigation.

The Washington Post’s Carol D. Leonnig and Robert Costa report that Mueller floated the idea during discussions with Trump’s lawyers last month. The threat, apparently, wasn’t well-received by Trump’s legal team, and negotiations broke down shortly thereafter.

Trump’s lawyers have consistently urged him not to sit down with Mueller, and so far it appears Trump is taking their advice. But if Mueller decides to press the issue and subpoena the president, he’ll be doing something extremely rare.

The last time a president was subpoenaed was in 1998 as part of the Clinton-Lewinsky saga. Before that, Richard Nixon was subpoenaed in 1974 to force him to hand over tape recordings and other materials related to the Watergate scandal.

I reached out to nine legal experts to ask if an independent counsel like Mueller has the constitutional authority to force Trump to testify. And if the fight between the White House and Mueller ends up before the Supreme Court, I wanted to know how they thought the Court would rule.

Their full responses, lightly edited for clarity and style, are below.

Jessica Levinson, law professor, Loyola Law School

I think it is highly likely that anyone, including the president, can be subpoenaed by a grand jury in a criminal case and must respond to that subpoena.

I consider this case all but settled by Clinton v. Jones. In that case, President Clinton sought to stay a sexual harassment case filed against him until at least the expiration of his term in office. The US Supreme Court ruled against President Clinton.

The Court found that a sitting president is not entitled to absolute immunity from civil litigation based on acts that occurred before taking office. Put another way, the president may only be entitled to immunity from civil litigation under exceptional circumstances.

Now, of course, Clinton v. Jones presents a somewhat different question than the one presented here with respect to President Trump. Clinton v. Jones involved a civil suit, not a criminal investigation. But it would seem that the need to ensure that the president cooperates with a criminal investigation is even higher than that need with respect to a private civil suit.

If this issue made its way to the Supreme Court, my guess is that the Court would rely on past precedent, including Clinton v. Jones, and find that the president must respond to a subpoena.

Diane Marie Amann, law professor, University of Georgia

The Supreme Court has never answered this precise question. Rules authorizing federal subpoenas — that is, court orders to testify or produce documents — do not expressly exempt the president. And in 1998, President Clinton was subpoenaed in connection with the independent counsel’s Whitewater-Lewinsky inquiry.

The bigger question, it seems, is the extent to which a president may challenge such a subpoena. As a legal matter he, like all witnesses, could invoke the Fifth Amendment privilege against self-incrimination or some other testimonial privilege. But the political risks of doing so are much greater for a president than for other witnesses.

“Trump will have a hard time getting the federal courts to quash a subpoena, but the court fight would certainly drag on for months and prevent the Mueller investigation from wrapping up anytime soon.” —Jens David Ohlin

Andy Wright, law professor, Savannah Law School

A subpoena of a sitting president to appear before a grand jury is legally sound if the investigation warrants it, but it raises constitutional sensitivities. Respect between co-equal judicial and executive branches suggests voluntary cooperation by the president is better than legal compulsion. Cooperation has largely been the tradition.

Presidents Ronald Reagan, Bill Clinton, and George W. Bush all gave evidence in federal criminal investigations. President Trump has flouted other separation of powers traditions, so we may be headed toward a constitutional showdown.

In July 1998, independent counsel Ken Starr served President Clinton a subpoena calling for his testimony before the grand jury in the Monica Lewinsky matter. President Clinton agreed to testify voluntarily under certain conditions including video conference testimony rather than a courthouse perp walk.

Starr accepted those conditions in return for Clinton’s agreement not to challenge the subpoena on constitutional grounds. That August, President Clinton testified via video conference.

Lisa Kern Griffin, law professor, Duke University

It seems clear from the Supreme Court’s unanimous decision in the 1974 Nixon case that the special counsel can subpoena a sitting president. The president is not “above the law.” And his general interest in confidentiality — absent some claim of secrecy related to national security — is insufficient to shield evidence or testimony essential to a criminal process.

The distinction between the Nixon case and the present situation is that the Court ruled the production of evidence (the Watergate tapes) was mandatory and it did not address witness testimony. Nonetheless, the reasoning seems to apply, and the president’s constitutional objection to the subpoena would more likely arise from his Fifth Amendment protection against compelled self-incrimination.

Susan Bloch, law professor, Georgetown University

The judiciary’s response to a president’s refusal to respond to a subpoena will depend on several factors. Specifically, we need to know:

1) Who is asking for the subpoena — a court, a special counsel, or Congress?

2) What are they seeking — testimony or documents?

3) Why are they seeking the information?

The final question is what happens if the president refuses to comply with a lawful court order enforcing a subpoena. Typically, contempt of court is the appropriate judicial response for an ordinary person defying such an order.

But what if the president defies such an order? Fortunately, we have never experienced such a constitutional crisis, but the general consensus is that impeachment is the appropriate next step. That, of course, is a political — not a judicial — remedy and thus depends on the willingness of Congress to go down that path.

“I think it would be enormously difficult to argue that President Trump need not respond to a grand jury subpoena.” —Jessica Levinson

Peter Shane, law professor, Ohio State University

Whether a sitting president may be subpoenaed may depend upon the nature of the subpoena (i.e., for documents or for testimony) and upon the issuing institution (i.e., a grand jury, a court, or a congressional committee).

With regard to judicial or grand jury subpoenas for either documents or testimony, the Supreme Court has established that presidential compliance may be constitutionally required. As the Court wrote 20 years ago in the Paula Jones case:

Sitting Presidents have responded to court orders to provide testimony and other information with sufficient frequency that such interactions between the Judicial and Executive Branches can scarcely be thought a novelty. President Monroe responded to written interrogatories, President Nixon … produced tapes in response to a subpoena duces tecum, President Ford complied with an order to give a deposition in a criminal trial, and President Clinton has twice given videotaped testimony in criminal proceedings. Moreover, sitting Presidents have also voluntarily complied with judicial requests for testimony. President Grant gave a lengthy deposition in a criminal case under such circumstances, and President Carter similarly gave videotaped testimony for use at a criminal trial.

I have little doubt that the courts would apply a similar balancing-of-interests approach in upholding a congressional subpoena for presidential documents. There is no clear authority, however, on compelling presidential testimony before Congress. The Justice Department has repeatedly asserted an absolute presidential immunity from congressional compulsion to testify.

Jens David Ohlin, law professor, Cornell University

Trump will have a hard time getting the federal courts to quash a subpoena, but the court fight would certainly drag on for months and prevent the Mueller investigation from wrapping up anytime soon.

Trump might try to assert executive privilege. For that argument, he’ll be stymied by the Nixon case precedents, which held that executive privilege is not absolute and can be overcome by a finding that the information is important and cannot be obtained elsewhere.

Since much of Mueller’s case will turn on Trump’s state of mind, that evidence — by definition — cannot be obtained from anyone else.

“But what if the president defies such an order? Fortunately, we have never experienced such a constitutional crisis, but the general consensus is that impeachment is the appropriate next step.” —Susan Bloch

Steven Duke, law professor, Yale University

A grand jury subpoena would almost certainly be enforceable against the president. The Supreme Court unanimously held in United States v. Nixon, 418 US 683(1974), that Nixon was required to comply with a grand jury subpoena for the Oval Office tapes.

The Court also unanimously held in Clinton v. Jones, 520 US 681(1997), that Clinton could be required to submit to a deposition in a civil case brought against him by Paula Jones.

In light of these two opinions, a majority Supreme Court opinion denying the enforceability of a grand jury subpoena for the testimony of President Trump is almost impossible to imagine.

Keith Whittington, politics professor, Princeton University

In the Watergate tapes case, the Supreme Court ruled that a federal court could issue a subpoena to a president to produce documents needed in a criminal investigation. It is generally accepted that presidents should normally comply with such an order.

The Court allowed the Paula Jones lawsuit to proceed against President Bill Clinton while emphasizing that judges should give the president the “utmost deference” while managing those proceedings. The Clinton experience has certainly led some to question the Court’s judgment that litigation would not unduly interfere with the ability of the president to conduct the nation’s business.

The problem of a judge issuing a subpoena to a president for testimony arose early in the nation’s history, and President Thomas Jefferson was emphatic that the president, not a judge, had to have the final say over whether, when, and how a president might offer testimony in a legal proceeding.

The proper independence of the three branches of government meant that the president could not be subjected to the command of a judge, otherwise “the several courts could bandy him from pillar to post … and withdraw him entirely from his constitutional duties.”

Presidents have voluntarily offered testimony while in office, and it is proper that they should do so. Given the unique constitutional responsibilities of the president, however, judges should be reluctant to attempt to require the appearance of a president for a legal proceeding.

If Congress believes that the president is abusing his discretion in this regard, then they have the means to put additional pressure on the president, ultimately including the impeachment power. If the people are dissatisfied with how the president is fulfilling the responsibilities of his office, then they have the means to change course through the ballot box.