Kamala Harris has made a dream start. But it's too early to count out Donald Trump | Jonathan Freedland

Trump unexpectedly closed a gap in the polls to a photo-finish in 2020. History has taught us it is far too soon to celebrate

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.

Jonathan Freedland is a Guardian columnist