Robert S. Mueller III, the special counsel investigating Russian interference in the 2016 election, appears to be looking into whether President Donald Trump obstructed justice.

It is not easy to prove that a president committed the crime of obstruction, but if publicly reported facts are accurate, Mr. Mueller is likely to find that he has a strong case against Mr. Trump.

Obstruction of justice is a serious offense that lay at the core of Bill Clinton's impeachment and forced the resignation of Richard Nixon. The Watergate-era cliché "The cover-up is worse than the crime" misses the point that the cover-up is a crime. Congress has made it a felony for any person — including the president — to "corruptly" interfere with a proceeding before a federal agency. Powerful evidence has emerged in recent weeks suggesting that President Trump did indeed interfere with the F.B.I. investigation of Mr. Trump's former national security adviser, Michael Flynn, which is part of the broader Russia inquiry.

So far, the case against Mr. Trump involves three key events. First, James Comey said that when he was the F.B.I. director, the president told him in a Valentine's Day chat, "I hope you can see your way clear to letting this go, to letting Flynn go." Two FederalCourts of Appeals have held that similar "I hope" statements can — depending on the context — support charges of obstruction.

Second, President Trump reportedly asked the director of national intelligence, Daniel Coats, in a private meeting in late March if Coats could get the F.B.I. to back off its Flynn probe. President Nixon's attempt to use the C.I.A. to shut down the Watergate investigation was one of the reasons the House Judiciary Committee voted for articles of impeachment on obstruction charges.

Even if none of those specific incidents would qualify as obstruction on its own, federal courts have said that an entire course of conduct can constitute obstruction. And whether Mr. Trump succeeded in his efforts is legally irrelevant, because federal law criminalizes attempted obstruction as well as successful obstruction. Nor does it matter whether there was an actual underlying crime.

The more difficult question involves intent — whether Mr. Trump acted "corruptly" when he sought to stymie the investigation. The president is the head of federal law enforcement, and prosecutorial discretion is a core element of executive power. No court would say that a prosecutor is guilty of obstruction for dropping a case because it is hard to prove, too expensive or even politically unpopular. How can a prosecutor's boss be guilty of obstruction by telling the prosecutor to stop?

The Harvard law professor Alan Dershowitz has argued that for just this reason, a president cannot be guilty of obstruction. Mr. Dershowitz notes, moreover, that the Constitution gives the president pardon power. President George H. W. Bush thwarted the Iran-contra investigation by pardoning former Defense Secretary Caspar Weinberger, and President Gerald Ford shut down the investigation of Nixon by pardoning him. Mr. Trump could easily end the Flynn investigation with a pardon, Mr. Dershowitz reasons, so he could also shut it down with a request to the F.B.I. chief.

But Mr. Dershowitz is wrong. The president's law enforcement discretion is not unlimited. He can't, for example, order prosecutors to enforce drug laws against black people but not white people. He also can't drop an investigation in exchange for a bribe. He can stop an inquiry for a wide range of reasons but not for "corrupt" purposes. The same is true with respect to pardons: While the president most certainly has this power, we know of no one who believes that the president can simply sell pardons for cash.

Mr. Mueller must now decide where to draw the line between "corrupt" intent and the legitimate exercise of prosecutorial discretion. He might start by looking at the Justice Department's own regulations, which prohibit prosecutors from taking part in an investigation in which they have a "personal or political relationship" with the subject. Under that standard, Mr. Trump's involvement in the Flynn investigation is immediately suspect. If Mr. Trump intervened to save a close associate and political ally from indictment, then the argument that he acted "corruptly" would be strong.

The president's constitutional responsibilities do make his standard of conduct more nuanced than that of an ordinary prosecutor. If Mr. Trump tried to block the investigation of Mr. Flynn because he believed that the inquiry would bring us to the brink of war with Russia, then that would suggest a noncorrupt intent. Mr. Mueller will have to ask not just what the president did but also why he did it.

By carefully considering this question, Mr. Mueller would be fulfilling his mandate, which extends to obstruction allegations arising out of his initial investigation. And even if he decides not to charge the president, his findings may show that Mr. Trump has abused his power in ways that would warrant impeachment, if not indictment.

Yet it is possible that Mr. Mueller will himself be thwarted before he reaches the end of his inquiry. Mr. Trump has reportedly considered firing the special counsel. If he does, the president will have interfered not only with the investigation of his campaign's Russia ties but also with the investigation into his own possible obstruction of the investigation. Covering up a cover-up would be yet another crime. In this, as in so many ways, Mr. Trump would break new ground.

Daniel Hemel and Eric Posner are professors at the University of Chicago Law School. This article appeared in The New York Times on June 15, 2017

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June 16, 2017