As Washington debates President Biden’s mental fitness, Americans are wondering what his exit might look like. The possibilities vary based on how and when he leaves, but the Constitution ensures that there’s a solid plan for whatever happens.

In the past 80 years, only two presidents have failed to complete their terms. The last, President Nixon, was fifty years ago, making this unfamiliar territory for most people. But like Nixon, an exit by Mr. Biden wouldn’t be unexpected.

America knows that Mr. Biden is, at 81, the oldest president in history, and multiple polls show that large majorities think he can’t do the job effectively. They would not be shocked — as they were by the assassination of JFK, the youngest president ever elected — if he cannot reach the finish line.

Some Republicans in Congress already want to invoke the 25th Amendment to remove Mr. Biden. That would require Vice President Harris and “a majority of either the principal officers of the executive departments or of such other body as Congress may by law provide” to act.

Mr. Biden would be declared “unable to discharge the powers and duties of his office” in writing to the president pro tempore of the Senate and the House speaker. This would make Mrs. Harris “acting president,” until Mr. Biden proved able to reclaim his job.

Bowing out of the presidential race while staying in office is also an option. In the mid-19th Century, that was common, but nothing similar has played out in recent memory, either. President Lyndon Johnson last did it in 1968, with the Democratic primaries underway.

After Kennedy’s death, Johnson’s withdrawal, and Nixon’s resignation, the parties chose their vice presidents to stand for the White House. Only Johnson triumphed, but they all gained a leg-up on other candidates.

Mrs. Harris would be helped by the bump a new president gets. In last Sunday’s NBC poll, 53 percent of registered voters viewed her negatively, almost double her 28 percent positive. If Mr. Biden only anoints her his successor, that unpopularity would linger and hurt her chances.

Prior to President Theodore Roosevelt in 1904, no former vice president who took over had won election in his own right. None of the four who acceded upon the death of a president before him were even nominated by their parties.   

The best role model for Mrs. Harris if she takes over could be President Arthur. Never elected to anything before the vice presidency, he was seen as a partisan hack. He was also mistrusted because President Garfield’s assassin, part of a rival GOP faction, had proclaimed, after firing the fatal shots, “Arthur is president.” 

Arthur soothed the heated divisions of the day to win America over with sound governing. However, suffering from a terminal kidney disease in secret, he made only a token effort for the 1884 nomination. President Ford’s efforts to end what he called the “long, national nightmare” of Watergate followed a similar path. He lost, but in the second-closest presidential election up to that time.

The window for a smooth, modern handoff to Mrs. Harris is closing fast. By March 19, the Democratic primaries will have awarded most of the available delegates. If Mr. Biden drops out of contention after that date, many of the supporters he won would go to the Democratic convention uncommitted.

A member of the Democratic National Committee, Elaine Kamarck, told ABC News that their delegates are only “pledged” to a candidate not “bound.” Party rules say they “shall in all good conscience” act in line with their voters, but there is no penalty for switching horses.

Governors Newsom of California and Whitmer of Michigan are among those seen as more popular in the party than Mrs. Harris. In the event they challenge her, the Democratic ticket might not be decided until the convention in late August.

Based on Mr. Biden’s defiant press conference following the special counsel’s report, he’s not considering an exit of any sort, but fate has a way of vetoing a president’s plans. Americans can prepare for the future based on the past, confident that while Oval Office occupants come and go, the Constitution endures.