What was the worst moment? Perhaps when one especially rambling sentence of Joe Biden’s ended in a mumbled, confused declaration that “We finally beat Medicare”, as if he were the enemy of the very public service Democrats cherish and defend. Maybe it was when the president was not talking, but the camera showed him staring vacantly into space, his mouth slack and open? Or was it when he was talking, and out came a thin, reedy whisper of a voice, one that could not command the viewer’s attention, even when the words themselves made good sense? For anyone who cares about the future of the United States and therefore, thanks to that country’s unmatched power, the future of the world, it was agonising to watch. You found yourself glancing ever more frequently at the clock, desperate for it to end, if only on humanitarian grounds: it seemed cruel to put a man of visible frailty through such an ordeal. In that sense, the first – and, given what happened, probably last – TV debate between the current and former president confirmed the worst fears many Biden supporters have long harboured over his capacity to take on and defeat Donald Trump. For more than 90 excruciating minutes, every late-night gag about Biden’s age, every unkindly cut TikTok video depicting him as doddery and semi-senile, became real. There was no spinning it, despite White House efforts to blame a cold. Joe Biden delivered the worst presidential debate performance ever. Expectations were rock bottom: all he had to do was turn up and show some vigour, reassure people that his marbles were all present and correct, and it would have been enough. The bar could scarcely have been lower. But Joe Biden could not clear it. And if the debate confirmed Biden’s limitations, it also served as a reminder of why those limitations matter. For one thing, Trump’s entire framing of this race is strong v weak: he offers himself as a strongman, against an opponent too feeble to lead and protect the US in an increasingly dangerous world. Purely at the physical level of what people could see and hear on their TV screens, the Atlanta debate reinforced Trump’s frame. But, no less important, Biden’s inability to deliver clear, intelligible statements meant Donald Trump’s lies went unchallenged. And there were so many, lie after lie after lie. Trump claimed Democrats favoured abortion at nine months, even if that meant killing babies after birth. He claimed the real culprit for the 6 January storming of Capitol Hill was not him, but Democratic former House speaker Nancy Pelosi. He claimed it was he who had lowered the cost of insulin, when it was Biden who did that. There were dozens more in that vein, an unceasing firehose of lies. But because CNN had made the baffling decision to have the hosts do nothing but read out scripted questions – never challenging any statement made by the candidates – it was left to Biden to hit back in real time. And he couldn’t do it. The post-match factcheckers stayed up into the early hours, attempting to set the record straight. But by then it was too late. In that sense, the debate was the 2024 campaign in microcosm. Trump is a liar, convicted felon and would-be dictator who plotted to overturn a free and fair election so he could cling to power, but he is set to return to the Oval Office because his opponent is too weak to stop him. As the former Obama administration official Van Jones put it after the debate, this is a contest of “an old man against a conman” – but the weakness of the former is allowing the latter to prevail. The simple fact that Trump spoke loudly and clearly and with, by his standards, relative self-discipline, coupled with the lack of interrogation from the moderators, granted him a plausibility he should have been denied. He is a failed coup leader, nationalist-populist menace and racist who would suck up to the world’s autocrats and throw Ukraine to Vladimir Putin’s wolves: he should be allowed nowhere near power. But because he was up against a man who could scarcely complete a sentence, he was presented as a legitimate option for the world’s highest office. The expectation must now be that, if he faces Biden on 5 November, Trump will win. He was ahead in all of the battleground states even before the debate and there is now no clear further chance for a reset. Thursday night’s head-to-head was supposed to be that moment. Indeed, that is why the White House opted to have the debate so unusually early: to allay fears about the president’s age and to reframe the race not as a referendum on Biden, but as a choice. That gambit doubly failed, making a bad problem much worse. So what now? Unfortunately, there is no letters-to-Graham-Brady mechanism in US politics, no equivalent to Westminster’s short, sharp defenestrations. Some imagine the likes of Hillary Clinton and Barack Obama having a quiet word, but Biden is a proud and stubborn man who feels he was passed over too long, including by those two. Perhaps Democratic leaders in the House and Senate could do it: given Biden’s decades-long attachment to Congress, he might listen to warnings that “down-ballot” candidates could suffer if he stays at the top of the ticket. But ultimately this will have to be his decision. He won his party’s primaries earlier this year, all but unopposed; the Democratic party’s nomination is his, unless he gives it up. Some say the only person who could ever persuade him to do that is his wife, Jill. But after the debate, she loudly congratulated her husband, albeit in a manner that reinforced the sense of a man well past his prime. “Joe, you did such a great job!” she said. “You answered every question! You knew all the facts!” Even those Democrats who concede Thursday was a calamity worry that a change now is fraught with risk. Biden could make way for his vice-president, but Kamala Harris is even less popular than he is – and Trump would relish mining the rich seams of sexism and racism that would open up. The party could throw it open to a contest fought out at its convention in August among the deep bench of next-generation Democratic talent – the Michigan governor, Gretchen Whitmer, her California counterpart, Gavin Newsom, and others – but that could be messy, bitter and rushed. None of the contenders has been tested under national lights, and Democrats would be turning their fire on each other when they need to be aiming at Trump. One thing Democrats agree on: Joe Biden is a good and decent man who has been an unexpectedly consequential president. But communicating is a key part of governing, and Biden has all but lost that ability. For the past year or so, Democrats have crossed their fingers and hoped the evidence taking shape before their eyes might fade, not least because any other course of action entailed great risk. After this disaster of a debate, they can no longer deny that inaction, too, is a risk – and, given the perils of a second Trump presidency, surely the much graver one. Jonathan Freedland is a Guardian columnist