During the two months I spent in the US this summer, I kept asking every journalist, academic and analyst I met one simple question: “Who will be the next president of the United States?” The response was usually the same. First there was a distinct hesitation, then they said “Well, probably Joe Biden, but …” What followed the “but” was a long list of concerns, partly about deeper trends but mainly about how old and frail the 80-year-old president looks. Often, the conversation ended with my interlocutor saying it would be better if Biden stood aside, to let a younger candidate turn the age card against the 77-year-old Donald Trump. Biden has been a good president of the United States. Although the retreat from Afghanistan was chaotic, he dealt with the Covid pandemic well and is handling the war in Ukraine fairly well. He is presiding over a remarkably vibrant economy, with New Deal-style public spending accelerating a green transition and creating jobs. But if he stumbles – physically, mentally or politically – during the gruelling marathon that is a US presidential campaign, and lets Trump back in, that’s the only thing Biden will be remembered for. In a recent NBC poll, Trump and Biden were neck-and-neck, scoring 46% each. Any one of a number of factors unrelated to the characters and performances of the two candidates might swing such a close election. In the country’s hyperpolarised media environment, many Republican voters simply don’t see that the economy is doing well. AI will add to the already high possibility of misinformation, with Vladimir Putin certainly eager to tip the scales in favour of Trump. Third candidate initiatives, such as the well-intentioned centrist initiative No Labels and the intellectual activist Cornel West’s progressive-environmentalist campaign, are likely to take more votes from the Democrats than from the Republicans. Most worryingly for the Democrats, there’s a trend of Black, Hispanic and other non-white voters shifting from Democrat to Republican, and especially from Biden to Trump. There are sociological and historical explanations for this, as well as the strange appeal of Trump himself, but there’s little doubt that Biden’s age and frailty play a role. In a recent poll, three out of every four Americans said Biden was too old for a second term – at the end of which he would be 86. Only half those asked expressed the same concern about Trump. I spoke to four individuals who had seen Biden at close quarters in recent months. They said he was mentally fine, but physically showing his age. One commented on the way his voice sometimes faded to almost inaudible at the end of a sentence. All this will be ruthlessly exposed in the 24/7 media coverage of a presidential election campaign. A single fall from an election rally stage by the Republican presidential candidate Bob Dole played a part in his defeat in 1996. And Dole was a mere spring chicken of 73, in a more sedate media environment. Biden comes with one other liability. Because of his age, unusual attention will be focused on his running mate, who may have to step into the hot seat. But the vice-president, Kamala Harris, is not a great electoral asset, and hardly convinces as a possible “leader of the free world”. Despite the initial excitement, she has seemed marginal to the presidency, has an approval rating lower even than Biden’s and has made almost no impact on the world stage. Of course, Trump has huge liabilities too – above all, the multiple lawsuits that are taking much of his time and campaign funding. If Jack Smith, the prosecutor in the central case concerning possible fraud in the 2020 presidential election, is as effective as some think he is, Trump might even be in prison when Americans vote next November. Yet, astonishingly to an outsider, there’s little evidence that these prosecutions have so far seriously damaged his election prospects. Obviously, there are also risks associated with Biden stepping aside at this late stage. Some observers expressed a concern that the fragile rainbow coalition of the Democratic party could tear itself apart if set to find a new candidate. A former congressman disagreed, pointing to the disciplining effect of the threat of Trump. Certainly, there are credible contenders of a younger generation, such as the Pennsylvania governor, Josh Shapiro (who would then be the first Jewish president), the Michigan governor, Gretchen Whitmer (who would then be the first female president), or California’s governor, Gavin Newsom. Not only would they turn the age card against Trump; they would also rejuvenate the image of the US in the world. At the moment, outsiders contemplate with astonishment what looks to us like a Brezhnevite gerontocracy in Washington. Biden, 80 going on 81. Trump, 77. The Republican Senate leader, Mitch McConnell, 81, freezing for half a minute like an old desktop with a bad internet connection. The former house speaker Nancy Pelosi, running again at the age of 83. One thing, however, is clear: the only person who can make this decision is Biden, with his wife, Jill. If it’s to happen, it would be best it happens fast, so younger candidates can declare themselves, raise sufficient funds and organise national campaigns, and then one of them can be selected and choose a credible running mate. “It must be before Thanksgiving,” one longtime observer of US politics exclaimed. That’s less than two months away. At this point, some American readers might be huffing, “Who’s this Brit telling us what we ought to do?” All I would say in reply is: sorry, but it’s not only your future that this contest will decide. There’s a bunch of interesting elections coming up on our side of the Atlantic: a crucial Polish election next month, which may determine the future of a fragile democracy; European parliament elections next June, which may see a sharp turn to the populist right; a British general election, which may see the post-Brexit UK returning to something vaguely resembling sanity; perhaps even a Ukrainian presidential election. None of these European elections will be as consequential for Europe as this American one. A second Trump presidency would be a disaster for the US. It would also be a catastrophe for Ukraine, an emergency for Europe and a crisis of the west. If Biden steps aside now, democrats everywhere will honour him, while the US Democrats can choose a younger candidate to see off Trump – and perhaps even inspire the world again with a sense of American dynamism. Timothy Garton Ash is a Guardian columnist This article was amended on 29 September 2023. An early version included reference to Dianne Feinstein among elderly politicians still holding office. Shortly after the article was published, the death of the California senator was announced.