Almost everything about the scene in a packed 10,000-seat basketball arena in Philadelphia on the evening of August 6th would have been unimaginable just six weeks ago. First, there was the sheer size and Swiftie-like zeal of a Democratic crowd waving their glow-in-the-dark wristbands, fired by belief that their ticket might actually win in November. Then there were the star attractions on stage: Kamala Harris, now formally ratified as the party’s presidential nominee, and Tim Walz, the governor of Minnesota, her new vice-presidential running-mate. Not since 1968 has one of America’s two major parties switched out a presumptive presidential nominee months before an election. On current evidence of the move’s effects, they might consider doing so more often.