President Richard Nixon at his desk at the White House in 1969. (AP Photo/File)
Ken Hughes is a researcher at the University of Virginia’s Miller Center and author of "Chasing Shadows: The Nixon Tapes, the Chennault Affair, and the Origins of Watergate."

President Trump is flashing his pardon power like a pocketful of Get Out of Jail Free cards. Not only is he musing on Twitter about pardoning boxer Jack Johnson, he actually pardoned Scooter Libby, an aide to former vice president Richard B. Cheney who was convicted of perjury and obstruction of justice.

The takeaway for former (and current) aides ensnared in the multiplying Trump investigations is obvious: If they protect him, he may protect them. Trump wouldn’t be the first president to offer clemency for complicity. President Richard M. Nixon secretly promised pardons to his top Watergate co-conspirators and dangled clemency over the heads of lower-level henchmen. But Nixon never delivered, and Trump will prove no more faithful. Because as it turns out, pardons are easier to promise than to provide.

With Nixon, it started almost as a joke. Even before the Watergate break-in, Attorney General John Mitchell was getting nervous. Nixon told Mitchell to leak some dirt on Daniel Ellsberg, who had leaked the secret analysis of the Vietnam War known as the Pentagon Papers to journalists. Unfortunately, that dirt came from testimony before the grand jury that was investigating Ellsberg. Leaking grand jury testimony is a crime for any government attorney, including the attorney general.

Mitchell protested that he didn’t “want to go to jail,” drawing laughs from chief of staff H.R. Haldeman and policy adviser John Ehrlichman. “Of course,” Mitchell added, “if I’m going to jail, I want to go in a hurry, so I might get a pardon.”

“Ha. You bet,” Nixon said.

“Don’t count on it,” quipped Ehrlichman, prompting more laughter.

Nixon was paranoid that the leak of the Pentagon Papers (which covered Vietnam policy up through 1968, the year he was elected president) was merely a prelude to a leak of his own darkest Vietnam secret — which had the potential to destroy him.

During his 1968 presidential campaign, Nixon had illegally colluded with the South Vietnamese government to block President Lyndon B. Johnson from starting peace talks before Election Day. Johnson found out about the collusion the week before the election from the CIA, NSA and FBI but chose not to go public for a variety of reasons.

Though never exposed during the election, fear of future disclosure left Nixon on edge. When Haldeman reminded him of a rumor that there was a top-secret government report on the 1968 negotiations locked in a safe at the Brookings Institution, Nixon snapped.

This, he feared, was the evidence that could unravel his presidency. He ordered aides to break into Brookings and steal the report. His top aides were understandably reluctant, so Nixon created “the Plumbers,” a secret police unit under his personal control, to commit the burglaries and illegal leaks that he deemed necessary to protect himself.

This started the spiral that led to the infamous Watergate break-in at the Democratic National Committee’s headquarters.

Nixon would have been happy to let the Watergate criminals go to prison. But he knew that a full investigation of their crimes could expose his, so he ordered a coverup. To keep the defendants’ mouths shut, the president’s co-conspirators arranged payments of hush money, raising their hopes of receiving executive clemency (pardons or commuted sentences) once the political heat died down.

The coverup held long enough to get Nixon reelected. Voters took the Watergate break-in seriously — 84 percent told the Harris Poll it was a “basic violation of civil liberties and individual freedom to put wiretaps in the opposition party’s headquarters” — but most (66 percent) didn’t think Nixon had anything to do with it. Pollsters didn’t even ask about a coverup — a sign it was working.

Everything started to fall apart early in 1973. One Watergate defendant revealed that the White House had offered him money and executive clemency in return for his silence. This development forced newspapers like the New York Times to define an unfamiliar term for their readers: “obstruction of justice — that is, interfering with the Justice Department’s inquiry.

The Watergate investigation quickly enveloped the White House. The Times soon reported that Haldeman and Ehrlichman, along with former attorney general Mitchell, could be indicted on a charge of obstructing justice. The possibility that obstruction went all the way to the top forced The Washington Post’s Bob Woodward and Carl Bernstein to explain another unfamiliar term: impeachment.

Nixon was in a corner. If his former aides testified truthfully, they would implicate him in the coverup. To protect himself, he needed them to perjure themselves. On the day after the Senate Watergate hearings began, Nixon secretly promised to pardon them no matter what.

There was “nothing more important” than “to keep me in this f—ing office,” Nixon told Haldeman. He could keep them out of prison. “I don’t give a s–t what comes out on you,” the president said. “There is going to be a total pardon.”

There was a problem with this promise: More than a century before, the Supreme Court had recognized that the president cannot pardon a crime before it is committed. The power to pre-pardon crimes would place the president above the law. This conversation alone would have justified impeachment.

In 1974, Haldeman, Ehrlichman and Mitchell were indicted on a charge of perjury, obstruction of justice and conspiracy to obstruct justice — all to protect Nixon and themselves. The indictment accused them of making “offers of leniency, executive clemency, and other benefits” to the Watergate burglars to secure their participation in the coverup. The president was named as an unindicted co-conspirator.

By July 27, the House Judiciary Committee’s chief counsel had identified 50 instances of obstruction of justice by Nixon, four of them involving illicit offers of clemency. That day, a bipartisan majority of the committee voted 27 to 11 to impeach Nixon for his role in the coverup. Impeachment by the full House looked inevitable. That same week, the Supreme Court ruled 8 to 0 that the president had to turn over 64 White House tapes for use as evidence in the criminal coverup trial.

One of those tapes — dubbed by one of Nixon’s attorneys as the “smoking pistol” — proved Nixon had been part of the coverup all along. On June 23, 1972, less than a week after the Watergate break-in, Haldeman warned Nixon that FBI investigators were on the verge of tracing a check from a burglar’s bank account back to the Nixon reelection campaign. The president approved a plan to have the CIA falsely tell the FBI that further investigation would blow vital agency secrets.

When the White House released a transcript of this tape, Republicans abandoned Nixon. Every Judiciary Committee Republican who had voted against impeachment now announced that they would vote for it on the House floor. That left the Senate, where Nixon needed just 34 votes to avoid conviction. GOP leaders thought he might get 20. If Nixon didn’t leave office voluntarily, he would leave involuntarily — and quickly.

With time running out, Haldeman requested pardons for everyone involved. Nixon’s lawyers were opposed. “If the president grants this pardon, he will be insuring his own trial,” one lawyer said. “He will be forcing it. The public has to have a head, and if the president takes the heads away, the public will have his.”

The morning headlines Aug. 8 said the end was near (“Nixon Resignation Appears Imminent” — Los Angeles Times). The afternoon’s said the end was here (“Nixon Quits; Will Explain Decision on TV Tonight” — Los Angeles Times Extra Late Final Edition).

Haldeman called Nixon at 4:18 p.m. He could not get through.

At 9:01, Nixon went on television to announce that his resignation would take effect at noon the next day. He said nothing about pardons.

Haldeman called the White House at 9:27. No one took his call.

The next day, after Nixon’s departure from the White House, he remained president for two more hours. As Nixon flew home to California on Air Force One, Haldeman telephoned the plane. It was the last chance for everyone who was depending on Nixon to protect them, as they had protected him, from the law.

Nixon didn’t take the call. It was his final decision as president. By noon, it was clear that the kind of president who would offer clemency for complicity was the kind who would back out of the deal.

This is why Trump aides facing criminal charges cannot count on presidential pardons. Pardoning them would add to the president’s own legal problems. It would open him to new obstruction-of-justice charges. It would free them to testify against him without fear of incriminating themselves. And it would lessen his chances of receiving a pardon himself after his presidency, should he need one.

Nixon did. Only “a full, free, and absolute pardon” from President Gerald R. Ford kept Nixon from being indicted. It also left his co-conspirators holding the bag. While Nixon could no longer be tried for making clemency-for-complicity offers, they were tried for merely conveying Nixon’s offers (among their other justice-obstructing acts). Some jurors told the judge they didn’t think it fair to try Nixon’s subordinates without trying Nixon.

Nevertheless, they found Haldeman, Ehrlichman and Mitchell guilty of all charges. The judge sentenced them to two to eight years in prison. Ford didn’t pardon them. In the end, the only co-conspirator to get a presidential pardon was the one who broke his promise to pardon all the rest.