Critics mocked John Tyler as “His Accidency” when he became president after William Henry Harrison died of pneumonia a month into his term in 1841. Harrison was the first president to die in office, and Vice President John Tyler’s ascendance clarified the rules of succession, but the experience failed to spur Americans to be better prepared for future accidents. Throughout most of American history, presidential candidates have chosen their running mates with little regard for the fact that a vice president is “a heartbeat away” from the Oval Office. For all the talk of selecting a potential successor who can handle the job of president on a moment’s notice, nominees have always used strategic criteria in picking their No. 2: Will the running mate carry a key swing state? Will he appease a disgruntled faction of the party? Will she complement the nominee with expertise in a particular policy area and shore up a perceived weakness?

Not this year. In Joe Biden’s selection of a running mate, what matters most is whether she — and he has made clear it will be a she — is prepared to take over should Biden die, become incapacitated or serve only one term. At 77, Biden is the oldest major-party presidential nominee in history, by a lot. Harrison, at 68, was the oldest man to assume the office until Ronald Reagan, who was two weeks shy of his 70th birthday. That record held until Donald Trump, who was closing in on 71 when he was inaugurated. Although Biden’s health appears to be good, the chances that he would die in office — purely as an actuarial matter — are greater than they were for Presidents Barack Obama (47 on assuming office), George W. Bush (54), Bill Clinton (46), the elder George Bush (64) or Trump himself.

Age need not be a defining characteristic for a president; everyone gets old differently. But the science is clear that mental decline often, though not always, accompanies one’s advancing years. Research suggests that 77 is well past the ideal age to assume such a difficult job; studies find that 16 to 23% of Americans over 65 show some form of cognitive impairment. In truth, Biden has long been known to ramble amiably in his off-the-cuff remarks, and Trump even as a young man would bluster his way through interviews. But partisans in both men’s camps are so invested in portraying their rivals as intellectually enfeebled that we can count on the question of mental acuity arising in the fall campaign — making the vice president’s readiness to serve more important than usual.

Historically, presidential nominees have paid lip service to concerns about their running mates’ competence, but political considerations have mattered at least as much. In the 19th century, the party bigwigs would anoint the running mate, usually with the aim of regional balance, pairing a presidential nominee from the Northeast with an understudy from the Midwest or the South. Or they might strive to balance a radical with a moderate or an establishment nominee with a firebrand.

The men they picked, however, often failed to convey the gravitas expected of a president, and those who inherited the office on the death of incumbents labored to prove their bona fides. After Tyler, Andrew Johnson, who took over when Abraham Lincoln was assassinated in 1865, never established his legitimacy; he barely survived an impeachment trial. Chester Arthur acquitted himself creditably after James Garfield’s assassination in 1881, but his party found him wanting and denied him the chance to seek election three years later. Throughout the 1800s, vice presidents seldom emerged as formidable presidential contenders in their own right.

As late as 1945, Harry Truman, who had been vice president for all of 12 weeks when Franklin D. Roosevelt died, could say that on becoming president, “I felt like the moon, the stars, and all the planets had fallen on me.” But the increased size and responsibilities of the executive branch following the New Deal and World War II afforded opportunities to ambitious vice presidents, such as Richard M. Nixon and Lyndon B. Johnson, to expand the role. In the nuclear age, moreover, concern over who had a “finger on the button” subjected vice presidential choices to increased scrutiny. After Dwight D. Eisenhower’s heart attack in 1955 and John F. Kennedy’s assassination in 1963, it became commonplace to note the vice president’s proximity to the Oval Office.

Accordingly, when presidents seemed destined not to complete their terms, the presence of a lightweight VP induced panic. The prospect that Nixon would be impeached during Watergate made it urgent that Spiro Agnew — no one’s idea of presidential timber — be replaced with a safer successor (Gerald R. Ford). Likewise, in 1992, when a sickly President George H.W. Bush vomited in the lap of Prime Minister Kiichi Miyazawa at a state dinner in Japan, pundits worried about the vice president he had picked four years before. Was Dan Quayle — a man known for such aphorisms as “What a waste it is to lose one’s mind” — really suited to be that heartbeat away? (Despite much speculation in the ensuing months, including that Bush would replace Quayle with a sure-handed pro like Defense Secretary Richard B. Cheney, Quayle stayed put, and Bush lost anyway.) Nor did many people consider Sarah Palin the most qualified replacement for John McCain in 2008 should he fail to complete a presidential term. In all of these cases, however, presidential nominees subordinated concerns about job readiness to the exigencies of winning the election at hand.

Even some of the vice presidents who turned out to be powerhouses within their administrations or viable presidential contenders afterward — Walter Mondale, Al Gore, Cheney, Biden himself — were selected for the No. 2 post at least in part for strategic reasons. Jimmy Carter needed Mondale in 1976 to appease liberals in the Democratic Party wary of Carter’s Southern populism; Clinton’s choice of Gore in 1992 signaled that he meant to break decisively with both Republican and Democratic dogmas; the younger Bush and Obama found in Cheney and Biden men with the foreign policy and Capitol Hill know-how that they themselves lacked.

Vice President Mike Pence projects aptitude in both governing and electoral appeal, but the main reason he will remain on the Republican ticket this year is surely because his presence reassures evangelicals and other conservatives who have at times doubted Trump’s fidelity to their causes.

Biden, by contrast, genuinely seems to be placing a premium on fitness for the job. This isn’t to say he has abjured all political calculation. On the contrary, knowing how important women are to the Democratic coalition, he boldly promised in a March debate — before he had the nomination locked up — to choose a female running mate. And the support he enjoyed from Black voters throughout the 2020 contests (and his career) has tilted the odds toward his choosing a Black woman. Still, within these confines, Biden is clearly focused on preparedness to govern more than is typical. “The most important thing,” he said back in the spring, “is that there has to be someone who, the day after they’re picked, is prepared to be president of the United States of America if something happened.”

Although Biden has always been famously gaffe-prone, his debate stumbles last year and some of his recent comments have provided an easy line of attack to his enemies on the left and the right. Biden and his aides even flirted with the idea of having him declare that if elected he would serve only one term. Though he ultimately rejected such a pledge, the possibility that he will step aside after four years still looms over his vice presidential choice. Biden has referred to himself as a “transitional” figure. The term implies that he doesn’t necessarily see himself having an eight-year presidency, which is usually what’s necessary to redirect the nation in a lasting way. Rather, having accomplished one important goal already — uniting the Democratic Party after five fractious years — he is intent primarily on dispatching Trump from the White House in November.

Biden’s running mate, then, whoever she is, won’t simply be a complement to the nominee. She will be a junior partner groomed to assume leadership of the party. And so more than ever before, voters will be treating the running mate not as an accessory to an image or an accent to a message, but as the heir apparent, the person who could assume the presidency in four years — or less — and set the course for the Democratic Party and the nation itself.

Greenberg is a history professor at Rutgers University. His books include “Republic of Spin: An Inside History of the American Presidency” and “Nixon’s Shadow: The History of an Image.”