After weeks of intensifying pressure, Joe Biden, the president of the United States of America, conceded to demands from within his own party to drop out of the November 2024 presidential election in which he was seeking a second term. Mr Biden’s decision is unprecedented: never before has a sitting president eligible for re-election quit so far down the nomination process. The last time an incumbent opted out of the race for a second term in the US was Lyndon B. Johnson in 1968. More importantly, the factors that led to this moment make this flux in US politics particularly significant. Throughout American history, sitting presidents unable to project their intent and plans for the future have either been in the late stages of their second term and, thus, constrained by their term limits; or, as in the case of Mr Johnson, unpopular because of their policies. In Mr Biden’s case, his low ratings appear to be driven in significant measure by the belief among a majority of voters that at 81 he is too old for another term. This set of circumstances has sent American politics lurching into uncharted territory, with consequences for the world.

Mr Biden immediately endorsed the vice-president, Kamala Harris, to lead the Democratic Party’s ticket. In just a few days, Ms Harris appears to have galvanised many previously unexcited voters and pulled in record funding for her campaign. She has won the endorsements of most possible challengers for the party nomination and appears poised to take on the former president, Donald Trump, of the Republican Party, in November. Suddenly, the age dynamic in the election has flipped. Mr Trump, 78, is now the oldest presidential candidate ever, while Ms Harris, 59, is amassing a rapid following on TikTok and other youth-friendly social media platforms. November promises to be a close race. Meanwhile, many Republicans have questioned Mr Biden’s fitness to complete the rest of his term, with some suggesting he should quit office. All of that has created a situation where Mr Biden’s stature as a leader in command of the US could come under scrutiny around the world, from friends and rivals alike. In a world ravaged by some of the most devastating and future-shaping wars in generations, uncertainty about the leadership of its strongest power could create a dangerous vacuum. Now, more than ever, mature heads must keep control, in the US and in capitals globally.