Why Joe Biden has dropped out of the presidential race - video explainer

Joe Biden has withdrawn from the race for the US presidency, an extraordinary decision upending American politics, that plunges the Democratic nomination into uncertainty just months before the November election against Donald Trump, a candidate he has warned is an existential threat to US democracy. Biden thanked the vice-president, Kamala Harris, in a letter announcing his decision, and later endorsed her as the Democratic nominee for president in a tweet.