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, dancing to a DJ’s tunes and fired by belief that their ticket might actually win the White House in November. Then there were the star attractions on stage: Kamala Harris, formally ratified as the party’s presidential nominee in an online delegate vote overnight, just over two weeks after Joe Biden’s decision to step aside. With her stood Tim Walz, the governor of Minnesota, Ms Harris’s newly announced vice-presidential running-mate. Not since 1968 has one of America’s two major parties switched out a presidential nominee so late in a campaign. On current evidence of the move’s effects, they might consider doing so more often.