By now, most of us are used to living in “unprecedented times.” But just how unprecedented is Joe Biden’s decision to withdraw from the 2024 presidential race, just over three months before Election Day?

Sometimes sitting presidents have decided not to run for re-election. But dropping out mid-campaign is incredibly rare. And it has never happened this close to an election.

Biden’s decision is a “massive earthquake,” said David Greenberg, a presidential historian at Rutgers University.

John J. Pitney Jr., professor of American politics at Claremont McKenna College, said: “There is no good analogy.”

The two political changes that came closest to Biden’s were the retirements of President Harry S. Truman before the 1952 election and of President Lyndon B. Johnson before the 1968 election.

Both Truman and Johnson had come to power after the death of a president and had served out their own terms; either could have run for a second term if they had wanted to. But after poor performances in their respective New Hampshire primaries in 1952 and 1968, they both dropped out of the race.

Truman withdrew from the presidential race on March 29, 1952, 220 days before Election Day. Truman, struggling with low popularity during the Korean War, dropped out of the race less than three weeks after losing the New Hampshire primary. (Primaries were relatively few at the time; in many states, party insiders controlled the nominating process.)

Ultimately, Adlai Stevenson II won the Democratic nomination, but lost the general election to Dwight Eisenhower, a World War II U.S. Army general who ran for the Republican nomination.

President Lyndon B. Johnson speaks in Washington on September 28, 1968. (AP)

Johnson dropped out of the race on March 31, 1968, 219 days before the election. Johnson—who had been made widely unpopular by another war, Vietnam—had not formally declared his candidacy and was on the New Hampshire ballot only as a write-in candidate. But after a poor showing and opposition from two strong candidates, Eugene McCarthy and Robert F. Kennedy, Johnson dropped out of the race.

Ultimately, Johnson’s vice president, Hubert Humphrey, was nominated, but he lost the general election to Richard Nixon.

Biden’s move comes much closer to Election Day — 107 days away — and after all Democratic voters have had their say in the presidential primaries.

There are a few other examples that look even less like Biden’s.

President Calvin Coolidge swore off his new term, but did so 15 months before it ended, in the summer of 1927, Greenberg said.

In 1976, Republicans went to their convention in Kansas City, Missouri, not knowing whether their nominee would be the incumbent president, Gerald Ford, or the conservative insurgent Ronald Reagan. Ford won the nomination but lost the general election to Democrat Jimmy Carter; four years later, Reagan won the nomination and the presidency.

How will the delegates decide?

The formal decision on Biden’s successor as nominee now rests with delegates attending the Democratic National Convention, being held in Chicago from August 19 to 22.

When voters cast their ballots during the Democratic primary season, they were technically choosing delegates for a candidate. The delegates assigned will formally decide who will succeed them as the nominee at the Democratic convention.

There are approximately 4,000 regular Democratic delegates earned through primary results and more than 700 “superdelegates” made up of party officials and elected officials.

The rules allow delegates to vote as they see fit, but because those delegates are almost entirely loyal to Biden, his support for Harris should carry a lot of weight, experts said.

Biden’s quick endorsement of Harris could even be enough to deter alternative candidates from seeking the nomination.

There will have to be a separate contest to select the vice-presidential candidate. The same system of delegate votes applies to the vice-presidential candidate as to the presidential candidate.

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