The best that can be said of Joe Biden’s stumbling debate performance was that it took place in June. If he were pressed to step down as nominee there would still be two months to go before the Democratic convention. For Biden’s loyalists, who have always moved swiftly to shut down any hint of dissent about his candidacy, Thursday night was a moment of truth. For more than a year, private conversations in Washington have been dominated by the president’s ageing. But the public omerta on that topic broadly held up. That cognitive dissonance has now collapsed. The story is now about whether Biden can be persuaded to step down.

The choice is his alone. Having crushed the Democratic nomination, Biden would be within his rights to ignore pleas to step aside. Potential alternative nominees, such as Gavin Newsom, California’s governor, and Gretchen Whitmer, Michigan’s governor, will be unlikely to speak out. The risk of being labelled a traitor and ruining their presidential chances would be too great. There is no such thing as a committee of party elders who can prevail on Biden to vacate the crown. He is the leader of the party. A tap on the shoulder from the younger Hillary Clinton, 76, or the much younger Barack Obama, 62, would risk backfiring.

Those who know Biden best say the only people who could influence him are his family, starting with the first lady, Jill Biden. Biden is a stubborn man. Most presidents are. Until Thursday night, he believed he was the only Democrat who could beat Donald Trump. Now it looks like he is on a course to defeat in November. As Biden’s mumbling, and often inaudible, performance went on, the prediction markets reacted in real time. By the end of the debate, one political betting market, PredictIt, gave Trump a 61 per cent chance of winning, having started the debate at 53 per cent. This put a number on what almost everyone was thinking.