The subpoena issued to Mike Pence this week by the special counsel investigating Donald Trump's failed bid to overturn the 2020 presidential election could irreparably sever relations between the former president and his onetime loyal vice president, if it causes Pence to testify against his old boss.

A half-century ago, Vice President Gerald Ford also faced a tipping point in his relationship with Richard M. Nixon, and decided to break with the beleaguered president.

Ford, who had loyally defended Nixon throughout the Watergate scandal as both House minority leader and vice president, finally told Nixon on Aug. 6, 1974, in the waning days of Nixon's presidency, that he could no longer defend him. In his memoir, "A Time to Heal," Ford recalled this seminal moment, which took place at a Cabinet meeting. Conscious that everyone in the room was looking at him, he told Nixon, "Mr. President, with your indulgence, I have something to say."

"Well Jerry, go ahead," Nixon replied.

"Everyone here recognizes the difficult position I'm in," Ford said. "No one regrets more than I do this whole tragic episode. I have deep personal sympathy for you, Mr. President, and your fine family. But I wish to emphasize that had I known what has been disclosed in reference to Watergate in the last twenty-four hours, I would not have made a number of the statements I made either as Minority Leader or as Vice President."

In its 2006 obituary for Ford, the New York Times reported that Ford told Nixon more directly, "I no longer can publicly defend you."

Ford told the president he had just come to the decision the day before, when the release of a secretly recorded June 23, 1972, phone call, known as the "smoking gun" tape, showed that Nixon had approved a plan for the CIA to block the FBI's investigation into the Watergate break-in. On Capitol Hill, fellow Republicans who had planned to support Nixon in an impeachment trial peeled away from him.

But even as Ford split with the president, he softened the blow by telling him, "You have given us the finest foreign policy this country has ever had. A super job. And the people appreciate it. Let me assure you that I expect to continue to support the Administration's foreign policy and the fight against inflation."

In some ways, Pence has already moved away from Trump. He told ABC News in November that Trump "endangered me and my family" on Jan. 6, 2021, when they were trapped in the Capitol as pro-Trump protesters stormed it. Some yelled "Hang Mike Pence!" because he resisted Trump's plan for Pence to use his position as president of the Senate to try to overturn Joe Biden's victory in the 2020 election.

But Pence has also given the president an out, blaming his conduct that day on bad legal advice. "Well, I don't know if it is criminal to listen to bad advice from lawyers," he said on "Meet the Press" last year, reflecting his caution not to alienate Trump loyalists with a full-out repudiation of the former president.

It's not clear yet whether Pence will comply with the subpoena. The Post has reported that Pence has privately expressed concerns about testifying against the former president because of executive privilege. But if he were to do so, that would probably mark a final break. Trump is already running for president in 2024; Pence is considering a run.

Like Pence, Ford had been a loyal vice president to Nixon, who nominated him to the office in October 1973 after Vice President Spiro Agnew resigned in his own scandal. Before that, Ford had been a key Nixon ally on the Hill as House minority leader. For example, in a May 1, 1973, call that was secretly recorded, Nixon vented to Ford that congressional Republicans needed to "start fighting back" after the resignations of two top Nixon aides.

"Anytime you want me to do anything, under any circumstances, you give me a call, Mr. President," Ford replied. "We'll stand by you morning, noon and night. . . . You've got a hell of a lot of friends up here, both Republican and Democrat, and don't worry about anybody being sunshine soldiers or summer patriots."

"Well, never Jerry Ford," Nixon replied. "But if you could get a few congressmen and senators to speak up and say a word, for Christ's sakes."

Trump also cajoled Pence, to a degree that might have made the bare-knuckled Nixon blush. As Pence recalled in his autobiography, "So Help Me God," he told Trump in a phone call on the morning of Jan. 6, 2021, that he didn't have the power to decide which electoral votes would count - and that he would be issuing a statement to Congress confirming that.

"The president laid into me," Pence wrote. "'You'll go down as a wimp,' he said. 'If you do that, I made a big mistake five years ago!'"

In Ford's case, he later explained why he had defended Nixon through much of Watergate: "I believed what I was told so my whole conduct as vice president was predicated on that personal trust."

At times, Nixon didn't return that trust. In May 1974, Newsweek described a meeting between Nixon and former New York Gov. Nelson Rockefeller (who would later serve as Ford's vice president). "Seated at his desk," the magazine reported, "the President asked Rockefeller contemptuously: 'Can you see Gerald Ford sitting in this chair?'"

Ford declined to comment on the report but said that he and Nixon "have had an excellent, personal, social, political relationship and I see no change whatsoever despite what some have speculated. We are firm friends. I admire him."

But around this time, Ford was starting to come out of Nixon's shadow. "One of his associates said this week that Mr. Ford no longer regarded his relationship to the President as that of a master and slave," the New York Times reported on May 29, 1974. "'He's been saying he's his own man, and now he's acting it,' he said."

At other times, Nixon expressed affection for his vice president. Just a couple of weeks after the Newsweek story, in a letter dated June 8, 1974, Nixon wrote to Ford:

"Dear Jerry, This is just a note to tell you how much I appreciated your superb and courageous support over the past difficult months. How much easier it would be for you to pander to the press and others who desperately are trying to drive a wedge between the president and vice president. It's tough going now, but history will I am sure record you as one of the most capable, courageous and honorable vice presidents we have had."