On Sunday morning, Sept. 8, 1974, the new president of the United States slipped out of the White House to attend St. John’s Episcopal Church across Lafayette Park, then returned to the Oval Office to address the nation. In office for only a month, Gerald Ford had decided, after consultation with Billy Graham and others, to extend a pre-emptive pardon to his predecessor, an act of mercy that Ford thought necessary for the well-being of both Richard Nixon and the nation.

That long-ago pardon raised questions that are relevant once again in the age of President Donald Trump.

Addressing the television camera that morning, Ford described the plight of Nixon and his family as “an American tragedy,” and the new president attributed his decision, in part, to his own religious convictions. Ford declared his belief that he himself would “receive justice without mercy if I fail to show mercy.”

The pardon of Nixon, who had yet to be charged with criminal conduct, met with widespread discontent. Ford’s newly appointed press secretary resigned in protest, and Ford took the highly unusual step of going to Capitol Hill, his old haunt as a member of Congress, to refute emphatically and unequivocally that his pardon of Nixon had been part of an arrangement made before Nixon’s resignation.

Newspaper editorial pages criticized the decision, and many Democratic leaders reviled Ford for the pardon. Sen. Edward M. Kennedy of Massachusetts remarked, “The tidal wave of national criticism over his pardon of Mr. Nixon should have shown the president that his instincts are clearly out of touch with the vast majority of the people of America.”

Sen. Walter F. Mondale of Minnesota introduced a constitutional amendment that would allow for a two-thirds vote in both houses of Congress to overturn future presidential pardons. “Mr. Ford’s pardon of former President Nixon proves that the issue of presidential accountability is still with us — and unresolved,” Mondale declared.

More than 44 years later, the notion of presidential accountability in the granting of pardons is more pressing than ever. To be clear, no responsible historian believes that Ford’s pardon of Nixon was self-serving. Most, in fact, have concluded that, despite popular outrage at the time, Ford did the right thing, even though the pardon probably cost him the election in 1976.

But self-serving is Trump’s stock-in-trade, whether it be his Trump International Hotel as a hangout for foreign potentates, his rollback of Dodd-Frank financial regulations, or taxpayer-funded visits to his golf properties. As Robert Mueller’s investigative noose begins to tighten around members of his administration, his family and Trump himself, it’s time to reopen the conversation about congressional checks on presidential pardons. (The president has already claimed the power to pardon himself.)

So far, Trump has bestowed pardons principally on conservative heroes: I. Lewis “Scooter” Libby, Dick Cheney’s chief of staff who leaked Valerie Plame’s identity as a CIA agent; Joe Arpaio, former sheriff of Maricopa County, Ariz., who illegally detained Latinos in conditions akin to a concentration camp; and Dinesh D’Souza, a political hack who pleaded guilty to using straw donors to contribute to a Republican Senate candidate.

As a growing number of erstwhile associates — Michael Kelly, Paul Manafort, Michael Cohen and others — negotiate plea agreements with prosecutors, Trump has opined that the practice of cutting a deal with prosecutors to secure shorter sentences, or no time at all, “almost ought to be illegal.” He has also hinted that pardons may be in store for those who refuse to cooperate, a suggestion that Roger Stone, Jerome Corsi and others seem to believe will shield them from prison time.

Trump’s exercise of presidential pardon, therefore, carries the potential to obstruct justice. If individuals refuse to testify about illegal activities with the understanding that they can do so with impunity, justice will be subverted.

Which brings us back to Ford’s pardon of Nixon. Although there was nothing unseemly about Ford’s action, the American people were denied a full accounting of Nixon’s misdeeds. (I believed at the time that, in the interest of transparency, the prosecutors should have continued their inquiry, even though the pardon meant that Nixon would never be brought to trial.)

The prospect of a congressional veto, as Mondale proposed in 1974, provides a good place to begin the conversation. The notion that both houses of Congress must override the pardon with two-thirds majorities would shield the process from unnecessary partisanship while at the same time providing a check on self-interested presidential authority.

In the decades since 1974, I can imagine other instances where a presidential pardon might have been open to scrutiny. Caspar Weinberger’s pardon by George H.W. Bush comes to mind, as does Bill Clinton’s pardon of financier Marc Rich on Clinton’s final day in office.

As Mueller’s probe intensifies, it’s time to shore up the checks on presidential pardons, lest they be used for self-interest or to thwart the engines of justice.

Randall Balmer, a former Minnesotan, is the John Phillips Professor in Religion at Dartmouth College and author of “God in the White House: How Faith Shaped the Presidency from John F. Kennedy to George W. Bush.”