President Trump. (Kevin Lamarque/Reuters)

Ken Gormley is president of Duquesne University in Pittsburgh and the author, most recently, of “Presidents and the Constitution: A Living History.”

President Trump’s firebrand attorney, Rudolph W. Giuliani, would be wise to warn his client to avoid, at all costs, flirting with the idea of pardoning himself. A little-known U.S. Supreme Court case from a century ago, Burdick v. United States, makes clear that acceptance of a pardon is a legal admission of guilt.

If Trump went down the self-pardon path out of contempt for special counsel Robert S. Mueller III’s investigation, he could paint himself into a dangerous corner: He’d admit being guilty of serious crimes that match the classic definition of an impeachable offense.

The Ford-Nixon era provides valuable clues for Trump’s lawyers.

In 1974, with the nation still haunted by the Watergate scandal, newly sworn-in President Gerald Ford discovered the 1915 Burdick case and fretted over its impact: What would it mean, legally, if he pardoned his predecessor Richard M. Nixon? The case involved a New York newspaper editor, George Burdick, who had been subpoenaed to testify in a federal grand jury about sources for a story on government fraud. When Burdick refused to cooperate and pleaded the Fifth Amendment, then-President Woodrow Wilson thought he’d outsmart him: The grand jury called Burdick back and had a presidential pardon waiting for him. How could a person invoke the Fifth if he’d been pardoned of any crime?

But Burdick refused to accept the pardon; he insisted that it made him appear guilty. The case went to the Supreme Court, which agreed with the editor. The justices wrote that a pardon carried with it “an imputation of guilt” and that acceptance of a pardon was a “confession” of such guilt.

Armed with copies of the Burdick opinion, the young lawyer dispatched by Ford, Benton Becker, met with Nixon at San Clemente, Calif. With Nixon’s lawyer, Herbert “Jack” Miller, present, Becker explained the Burdick holding in detail, and put Nixon on notice that if he accepted a pardon from Ford, it would constitute a legal admission of guilt.

On the 25th anniversary of the pardon, in 1999, at a program held at Duquesne University in Pittsburgh, Becker and Miller confirmed this story. Indeed, Miller (with the permission of the Nixon family) disclosed for the first time that Nixon resisted accepting the pardon, because he did not wish to admit guilt. It was only after a heated debate with his lawyer that Nixon accepted the pardon, despite its damning legal implications.

For decades, Ford carried around a scrap of paper in his wallet, with a citation to the Burdick case printed on it. He expressed frustration to me and to others that the media and the American public never fully appreciated that he’d gotten what the country wanted most — a legal admission of guilt from Nixon with respect to Watergate crimes. Even today, this fact remains largely lost to history.

Although Giuliani and Trump’s legal team might try to disavow it, the Burdick case remains good law. Nor is there any reason to believe that today’s Supreme Court would overturn this century-old precedent.

Indeed, Trump’s own Justice Department’s rules relating to the awarding of pardons mirror that precedent, stating that “a presidential pardon is ordinarily a sign of forgiveness and is granted in recognition of the applicant’s acceptance or responsibility for the crime.”

Three presidents in American history have escaped the clutches of the impeachment process — Andrew Johnson, Richard Nixon and Bill Clinton. In each case, conviction did not occur largely because there was no conclusive proof — or admission — of criminal conduct. (In Nixon’s case, even though articles of impeachment were reported to the full House, he resigned before criminality could be established and before impeachment proceedings could get underway.)

If Mueller’s prosecution team concludes that President Trump is guilty of obstruction of justice, collusion with Russia or any other federal offense, and if Trump seeks to pardon himself, then Trump would do something that no other American president has done: Make a legal admission of guilt, while in office, relating to criminal charges that are serious and involve conduct that presumably put the country at risk.

All chief executives under the spotlight of suspicion have pushed back from special prosecutor probes. That is not unusual. Yet political and tactical maneuvering, if taken too far, can cross a line that leads to catastrophic results.

Giuliani would be well-advised to tell his client to save his presidential pardons for those prepared to admit to crimes.

And for those who don’t intend to reside in the White House.