Joe Biden is not entirely on top of things lately. Whether because of age-related mental lapses or a more characterologically fundamental indifference to detail, he has in recent weeks asserted his health care plan involves $1,000 copays (it’s actually $1,000 deductibles), instructed debate viewers to “go to Joe … 3-0 … 3 … 3-0 [to] help me in this fight” (he meant “text the word Joe to the number 30330 to donate to my campaign”), said that “poor kids are just as bright and just as talented as white kids” (he meant “wealthy kids,” he explained), and twice claimed to have met with Parkland shooting survivors in D.C. while he was vice president (he did meet with some Parkland students after the attack, but it happened more than a year after he and Barack Obama left office).

Despite this, Biden maintains a double-digit lead in Democratic primary polls and a near-double-digit lead in polls that match him up head-to-head against Donald Trump. If trends hold—a big if, etc., etc.—Biden would be sworn in as president in 2021, whereupon he will immediately become the oldest person to have ever occupied the office.

Trump himself is currently the second-oldest president, and the theory behind Biden’s campaign seems to be that at least he’d be more fit for office than our current spaced-out POTUS, whose own possibly age-related struggles exist mainly as a side note to his rejection of the basic duties of the job in favor of cable news–based self-gratification and personal enrichment. But what would it be like to have a president who is really trying to be president, implementing a policy agenda motivated by political beliefs and principles, while still being somewhat out to lunch?

For answers, we can look to Ronald Reagan’s term in office. Reagan, who still holds the distinction of being the oldest president, was widely believed to be struggling with the effects of age even before he was diagnosed with Alzheimer’s in 1994. While there’s no direct evidence that Reagan suffered the symptoms of the disease until after his tenure in the White House, he was never known as a hands-on detail guy or steel-trap mental genius. Even Reagan’s sympathetic authorized biographer, Lou Cannon, wrote that the 40th president was “inattentive, unfocused and incurious” about the details of his job, fell asleep during meetings, and was known to recount the plots of movies as if he was describing things that had happened in real life.

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University of Texas historian Jeremi Suri is the author of the The Impossible Presidency: The Rise and Fall of America’s Highest Office, a study of the inhuman workload the chief executive role imposes on those who take its responsibilities seriously; he also contributed a chapter about Reagan to a new collection called Presidential Misconduct. According to Suri—and accounts of Reagan’s presidency given by Cannon and others—the ex-actor’s White House worked best when experienced and loyal staffers (in particular first-term chief of staff James Baker) were able to lay the groundwork for him to use his famous interpersonal skills to close deals with Democratic leaders in Congress and, later, Soviet leader Mikhail Gorbachev.

“The superficial way to tell the story is that Reagan and [Speaker of the House] Tip O’Neill learned to like each other, but Baker and others did a lot of good work to build a space of compromise that could bring Tip O’Neill and Ronald Reagan together,” Suri says. At its worst, though, the Reagan administration was an aimless place in which crises like the AIDS epidemic and the National Security Council’s Iran-Contra scheme were exacerbated and/or created by a leadership vacuum.

Suri observes that Reagan’s limitations didn’t just mean that he failed to follow complicated discussions on a moment-to-moment basis, but that he often lacked the long-term attention span to resolve disputes between administration factions and overcome external resistance to his agenda. “Ninety-five percent of the job, honestly, is banging heads together and grinding stuff through, and that takes enormous energy, patience, resilience, and focus,” Suri says. Reagan, in his telling, didn’t have the endurance to bang those heads together in a way that could translate his campaign themes—for instance, his promise to build up the military—into a practical agenda. “Reagan came in thinking, OK, I’m going to strengthen the military. But what that meant he hadn’t really articulated. A lot of people thought that meant we were gonna build more nukes, but he hated nuclear weapons.” (He’d later, in fact, develop great affection for the 1983 anti-nuke Matthew Broderick film WarGames.) The result, Suri says, was a Defense Department and a White House staff whose nuclear strategies were at odds, frustrating NATO allies and causing confusion in negotiations with the Soviets.

A less-than-on-the-ball Biden, Suri believes, could face similar problems in both foreign policy—where he’s yet to back up his promises to revive the Iran nuclear agreement and Paris climate accord with explanations of how he’d actually do so—and domestic policy, on which he might feel pressure from Obama administration veterans and members of the Democratic Party’s progressive wing to hire staffers and advisers whose views on, say, criminal justice issues don’t necessarily sync up with his own. He’s also been relatively vague about what his top priorities would be in office. While Reagan always made clear what his most urgent goals were—projecting the aforementioned strength abroad while cutting taxes and eliminating regulations at home—Biden has been less specific about what it is, aside from generally protecting Obama’s legacy, that he would use the powers of the presidency to do after being inaugurated.

On the other hand, the former Delaware senator has vastly more experience than Reagan did in writing and passing legislation—more such experience, in fact, than any president has had since Lyndon Johnson. “I do think Biden knows the legislative process and has the relationships and shows the inclination,” Suri says. “That’s a space he seems to work in well. But I’m skeptical about him managing the bureaucracy and the levers of government beyond that.”

Which is a big job regardless of how old you are. “I think the evidence is pretty clear that this is almost an impossible office for anyone to handle even if they’re fully healthy and young,” Suri says. “Obama was healthy and intelligent, and look at what the office did to him.”