Has Kamala Harris got what it takes to beat Donald Trump?

This renewed sense of Democratic presidential enthusiasm comes with a dollar sign attached. According to the Harris campaign, the vice-president raised more than $80m (£62m) in new donations in the 24 hours since Mr Biden’s announcement – the biggest one-day total of any candidate this election cycle. That, along with the nearly $100m she inherits from the Biden-Harris fundraising coffers, gives her a firm financial footing for the campaign to come.

Ms Harris, if she becomes the nominee, also defuses one of the most effective attacks the Republicans have levelled against their opponent: his age.

For months, the Trump campaign has been pounding Mr Biden for being feeble and easily confused – characterisations that were reinforced for many Americans after the president’s halting debate performance four weeks ago.

The vice-president, at age 59, will be a more energetic campaigner and able to make a more coherent case for her party. She could also turn the 78-year-old Trump’s age against him, as he would become the oldest person ever elected president.

Ms Harris may also be able to shore up support from black voters, who polls indicate had been drifting away from Mr Biden in recent months. If she can combine that with more backing from other minorities and younger voters – Barack Obama’s winning coalition from 2008 and 2012 – it could help her gain ground against Trump in the handful of swing states that will decide this year’s election.

Her background as a prosecutor could also burnish tough-on-crime credentials. While her law-enforcement resume was a liability when she ran for the Democratic presidential nomination in 2019 – and led to derisive “Kamala is a cop” attacks from the left – it could help her in a campaign against Trump.

The vice-president has also been the administration’s point person on abortion, which has proven to be one of the most potent issues for motivating the Democratic base in recent elections. Mr Biden, by contrast, sometimes had been a reluctant advocate on the issue, hampered by a past record of supporting some limits on the procedure.

“I think she reminds suburban women across the country, particularly in those battleground states, of what's at stake with reproductive rights,” former New York congressman Steve Israel, who headed the Democratic Congressional Campaign Committee, told the BBC’s Americast podcast.

"We have established a fundamental reset in the campaign.”