The bar was set low for him, but Donald Trump still didn’t manage to clear it. The former president has faced growing concerns from within his party that he no longer has the stamina, stylistic novelty or mental acuity to defeat Kamala Harris, even as polls narrow in the final weeks before November’s election. He did little to dissuade those fears on Tuesday, when he delivered a rambling, incoherent, lie-filled exposition of his own grievances in his first debate matchup against the US vice-president – a crucial moment in the presidential contest that proved to be a disastrous humiliation for him. Harris, after a beat or two of appearing nervous as the debate began, set about a methodical attack on Trump that repeatedly named him selfish, dishonest and weak. She goaded him with attacks on his ego and his potency – including a transparent but wildly effective remark about people leaving his rallies early from exhaustion – that caused him to explode into paroxysms of nonsensical woundedness. Trump, who initially had tried to land attacks on inflation, was soon reduced to racist ramblings, tangential defenses of his past remarks and records, attacks on Joe Biden, who is not running against him, and old lies about infanticide, fantasies about “world war three”, weird comparisons of the United States to Venezuela, a morbidly racist fantasy about immigrants killing and eating white people’s household pets, “transgender operations on illegal aliens in prisons”, and his false claims to have won the 2020 election. Trump has had bad debate performances before – including against Hillary Clinton, whom he ultimately defeated in 2016. But there is reason to suspect that his performance on Tuesday may genuinely harm his re-election chances in ways that will be difficult to recover from in the dwindling number of days before voters cast their ballots. The debate, the first since Harris replaced Biden at the top of the ticket, was widely anticipated to be a contest over who could best define the Democratic nominee, a figure many mainstream voters say that they do not know much about. But Trump failed to convincingly land attacks on Harris, and instead he spent much of the night arguing on the turf that his opponent chose for him. There was no bait she offered him that he didn’t take. He kept relitigating his past remarks, exploring grievances against former enemies living and dead, claiming to have been wronged by vast forces beyond public accounting, and indulging in references to elaborate conspiracy theories about his own righteousness and the nefariousness of his enemies. It is not a version of Trump that has appealed to voters in the past. In 2020, in his first debate against Biden, Trump’s aggressive, frantic, shouting performance led the then candidate Biden to say with exasperation: “Will you shut up, man?” It was a moment of vicarious release for the American audience, who were able to see their own exhausted frustration with Trump channeled through an on-screen proxy. In a less spontaneous, more intentional performance on Tuesday, Harris repeatedly cast Trump as a tiresome relic of an unappealing past – and herself as a refreshing break that can carry the country into the future. Harris has been criticized by some in her own party for having an insufficiently clear policy agenda, but this is more the argument for her candidacy than any white paper her staff may issue: she wants to meaningfully break from the Trump era – not in a transitional period or interregnum, as Biden did, but by ushering in a new generation of political leadership that can leave Trump more decisively behind. Her debate performance was meant to convey the message that Trump’s imbecilic cruelty was not so formidable, not so scary, not so inevitable as Americans have resigned themselves to thinking it was – that it was laughable, small – and that it could be defeated. Harris’s attacks hit Trump where it hurt: his manhood. She repeatedly referred to American military leaders who had worked with Trump, whom she said had described him to her as “a disgrace”. She recast his affinity for strongmen dictators around the world as less a fellowship than as a naive, even childlike fandom, suggesting his respect for them is not reciprocated and that Vladimir Putin “would eat [him] for lunch”. A friend I watched the debate with, an expert in psychoanalysis, described Harris’s tactics as a “symbolic castration”. Trump reacted almost as if it were the real thing. He bellowed and ranted with offense, his anger giving credence to Harris’s depiction of him as thin-skinned and weak. Perhaps the highlight of the night came in Harris’s response to the debate’s second question, about reproductive rights. Trump, whose position on abortion changes about as frequently as the tides, claimed his contribution to the end of Roe v Wade was only fulfilling “what the people wanted”. Harris responded with an eloquent, impassioned litany of the material deprivations and indignities forced upon those who seek abortions – from women who struggle to afford the children they already have to those who have been victims of rape. “They don’t want that,” Harris said of this state of affairs. “That is immoral,” she said of the laws she calls “Trump abortion bans”: a moving reversal of the anti-choice movement’s historical claim to the moral high ground. The moment was a potent reminder of her strengths as a candidate over Biden, whose answer on abortion in the June debate was barely coherent but thoroughly degrading to American women. Finally, it seems, the Democrats are willing to embrace their strongest issue, and American women’s interests might be represented on the national stage with something like the gravity and respect that they deserve. Early in the night, in a rare moment of lucidity and honesty, Trump spoke of his own policy plans. “I’m an open book,” he said. “Everyone knows what I’m going to do.” And it was true, though perhaps not in the way he meant it. Trump is, by now, a thoroughly familiar and predictable character, one you can always rely on to pursue narcissistic gratification and vulgar self-interest. If he’s an open book, Americans already know the ending. The Harris campaign is betting that they want to hear a different story. Moira Donegan is a Guardian US columnist