Joe Biden’s legacy will “rest in part” on the outcome of the presidential election campaign that has been energised by his withdrawal, an expert on American politics has said.

Leslie Vinjamuri, director of Chatham House’s US and Americas programme, said the “historic” moment had “unleashed energy into an election that for months had felt routine, and seen the lowest levels of enthusiasm among a worn-down electorate”.

Ms Vinjamuri, a professor of international relations at SOAS, said: “The President’s announcement has created the challenge of a generation for the Democratic Party – to identify and unite behind a new nominee, and running mate, and to drive confidence and enthusiasm among registered Democrats, and undecided voters, especially in swing states.

“A new momentum in the Republican Party around its America First agenda has placed extra pressure on what is already an urgent problem for Democrats.”

She added that Mr Biden’s legacy would “rest in part on the outcome of the 2024 election”.

Ms Vinjamuri said: “If Democrats succeed in defeating Donald Trump, his decision will be seen as key to securing the future for the country of a liberal, democratic leader and – quite possibly – of defending truth in politics.

“If Democrats lose, Biden’s legacy will undoubtedly be tarnished, and the President blamed for delaying a crucial decision just long enough to deny the Democratic Party the possibility of victory against a leader with autocratic instincts.”

Dr Mark Shanahan, associate professor and expert in US politics at the University of Surrey, said Mr Biden’s candidacy had “been a slow car crash since his abysmal performance in the first presidential debate”.

He added that the President had “bowed – stiffly – to the inevitable”.

Dr Shanahan added: “He was never going to give Trump the satisfaction of quitting before the Republican National Convention, so the decision to withdraw today is really no great surprise.

“Biden has endorsed Kamala Harris – the ultimate continuity candidate and presents the incumbent President as King/Queen-maker in the race.

“He will now hope the party falls into line and gets behind the VP’s candidacy. But that’s not a certainty, and the next 24 hours could yet throw up a rival candidate with the possibility of a super-primary over the coming weeks, or even an open convention in Chicago in August.

“Whatever happens, the whole dynamic of the presidential race has now changed. Harris, or any new Democrat candidate will bring new energy to the race – and suddenly, it’s Donald Trump who’ll look old, and sound less coherent by comparison.”

Dr Shanahan said Ms Harris’s choice of running mate “will be crucial” if she wins the Democratic nomination.

He added: “It will most likely be a mid-western male, and the likes of Pennsylvania Governor Josh Shapiro may already be limbering up in the wings.”

Dr David Brockington, a lecturer in American politics at the University of Plymouth, said the “gamble” taken by Mr Biden’s campaign team to take part in an early debate had “ricocheted back on them”.

The academic added: “In the view of many, this only confirmed critiques around Biden’s age, and cast a shadow his campaign found it impossible to escape.

“In endorsing Vice President Harris, Biden has warded off a potentially chaotic, contested convention which many believe torpedoed the chances of Democrats in 1968.

“That convention was also held in Chicago. Kamala Harris goes into this Chicago convention as the presumptive nominee, presumably with a unified Democratic Party behind her.”