Everything is going right for her and wrong for him. Kamala Harris has the encouraging poll numbers and, more precious still, the momentum. Donald Trump has the serial errors, the maudlin introspection and wobbling campaign team. In less than three weeks, the Democrats have pulled off one of the most extraordinary turnarounds in US political history, replacing a candidate who was shuffling towards near-certain defeat with one apparently soaring towards possible victory. And yet, even as Harris speaks of bringing the joy, contained within is a lurking danger – a peril that should be all too familiar.

The sources of joy are not mysterious. Democrats are heading to Chicago for a convention that will feel like a party but was set to be a wake. Before 21 July, they were tied to Joe Biden, a man whose presidency has proved far more consequential than most predicted but who was on course to lose and lose badly in November. His passing of the baton to his number two has gone better than anyone had any right to expect.

All but seamlessly, the campaign has switched over – equivalent to rebuilding a plane in mid-air, say seasoned election hands – and the candidate herself has taken to the task with unexpected ease. Twenty years younger and a whole lot more vigorous than her opponent, she has turned what had been Trump’s most potent weapon against Biden – age – against Trump himself. He is now the candidate of the past, she the face of the future. Never mind that Harris is a senior member of the present administration, she has shaken off the burden of incumbency – currently a negative in most democracies across the world – and cast herself as the turn-the-page option, aided by a powerful slogan: “We’re not going back.”

The evidence that it’s working is in the headline poll numbers, which show her edging ahead in the very battleground states where Biden had been trailing. Almost overnight, she is winning back the voters who propelled Biden to victory in 2020........

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