President Biden is suddenly playing coy about seeking reelection, raising the prospect of a second consecutive one-and-done presidency — something America hasn’t seen since the pre-Civil War years, when building a coalition to govern a fracturing nation frustrated one weak president after another.

Speaking to Scott Pelley of “60 Minutes,” Mr. Biden said it’s his “intention” to seek reelection. “Is it a firm decision that I run again? That remains to be seen.” He’ll have to decide soon to give other Democrats time to build the organizations they’ll need to compete.

Look for that decision after the midterms. Should Democrats lose the House or Senate, Mr. Biden will face Capitol Hill Republicans with a mandate to oppose his agenda. As William F. Buckley said, “A conservative is someone who stands athwart history yelling, ‘Stop!’”

Should Democrats hold one or both houses, though, Mr. Biden might feel compelled to stay. Otherwise, the moment he announces he isn’t a candidate, he’s a lame duck. This is the mistake President Theodore Roosevelt made in 1904, choosing to count the term he won and the completion of President McKinley’s unfilled term as eight years.

In the 20th Century, the only presidents to announce they wouldn’t seek reelection were following Roosevelt’s example, though they didn’t do so on Election Night as he did, which his wife Edith recognized as a strategic blunder.

Presidents Coolidge, Truman and Johnson all rose to the top job upon the death of their predecessors, too, and chose to respect the spirt of President Washington’s two-term precedent even before it was codified by law by the 22nd Amendment.

President Arthur might also have joined their ranks, but he made only a half-hearted attempt at reelection after inheriting the job from President Garfield in 1881, knowing that he was dying from a kidney ailment and couldn’t complete a second stint.

To find a one-termer who declared on day one that he’d lay down his burden after four years, you’d have to go back to President Hayes in 1876 who, unlike Mr. Biden, came into office in a disputed election, having lost the popular vote but won the Electoral College after the Supreme Court awarded him three disputed states.

One-and-done had been the norm leading up to the Civil War. Following President Jackson — derided as King Andrew by the Whigs — Americans recoiled at an imperial chief executive, and Congress clawed back power in the constitutional tug-of-war between branches.

In 1844, President Polk was first to declare his intention to serve just four years. Known as “Young Hickory” as a nod to Jackson’s “Old Hickory,” he was successful, but it was downhill from there leading up to President Buchanan and secession.

Buchanan provides the closest analogue to a long goodbye. Like Mr. Biden, he had a resume of service in Washington, but came to power as an old man, long after the friends he might have called upon to strike deals and govern had left the scene.

Thomas Balcerski, author of “Bosom Friends: The Intimate World of James Buchanan and William Rufus King,” tells me, “It would be unprecedented for Joe Biden not to seek a second term. He is our oldest president, however, and like a James Buchanan, he may well come to the conclusion that he is not up to the task for a second term.”

Mr. Balcerski sees this as unlikely. “Instead, Biden — a student of history — will seek a second term, if, as he puts it, fate allows.” His party will, of course, have a say; 75 percent of Democratic voters in a July CNN poll saying they want someone other than him in 2024.

In the 20th Century, the American presidency has evolved into a stronger office. But the same divisions that frustrated Buchanan exist today, calling for a strong hand on the wheel and a chief executive who can rise above factions and unite the nation.

Mr. Biden may yet be that man, but time is running short for a decision and — as in the days of Buchanan, his fellow Pennsylvania-born president — America’s fate will hang on whether he is up to the job.