For a US presidential candidate, the choice of running mate is often nakedly political. Who will clinch a swing state or put an unpromising one into contention? Who will please a wing of the party that remains cool on the main name? For Joe Biden, the test was more substantive. At 77, he needed someone who could share the burdens of office and, in extremis, replace him.

With her national experience, Senator Kamala Harris was better-suited than the mayors and governors who were touted. Unlike Susan Rice, the former national security adviser, she is steeped in domestic politics. For Donald Trump, the beleaguered president, a Biden-Harris ticket will be difficult to overcome.

Her elevation also reinforces the progressive trend of the Democratic party. Mr Biden was running to the left of Barack Obama, his old boss, even before the coronavirus pandemic and the race protests. On tax, healthcare and climate change abatement, he is open to a larger role for the federal government. He saw off Bernie Sanders, the Vermont leftist, by co-opting some of his ideas. Because he is so outwardly conventional, the ambition of his platform is sometimes missed.