By Anthony Zurcher, BBC News
Kamala Harris; Donald Trump. Photo: Brendan SMIALOWSKI and Patrick T. Fallon / AFP
Less than 24 hours after Joe Biden announced he was abandoning his re-election bid, the path for Vice-President Kamala Harris to secure the Democratic presidential nomination is clearing.
That may end up being the easy part. The most formidable challenge - defeating Republican nominee Donald Trump in November - is still to come. Her elevation to the top of ticket would bring new strengths for the Democrats, but it also exposes weaknesses that were less of a concern with Biden.
According to recent polls, Harris trails the former president slightly - a roughly similar position to the one Biden found himself in before his historic announcement. But there may be more room for those numbers to shift as we move from a hypothetical matchup to a very real one.
For at least a moment, Democrats have a jolt of energy after more than three weeks of intense hand-wringing over the president's fitness and ability to sustain his campaign.
All of Harris's leading potential rivals for the nomination have endorsed her, as has former Speaker of the House Nancy Pelosi - who remains one of the most influential players in Democratic politics.
This is, still, shaping up to be a tight race in November - a condition that reflects deep partisan trenches in American politics and the distaste many voters have for Trump as a candidate.
The vice-president's primary challenge - and opportunity - will be to capitalise on this Trump aversion, attract centrist voters in key swing states and energise the Democratic base, which was in the past few weeks swinging towards despair, to match the enthusiasm many on the right hold for the former president.
A reset?
This renewed sense of Democratic presidential enthusiasm comes with a dollar sign attached. According to the Harris campaign, the vice-president raised more than US$80 million (NZ$134m) in new donations in the 24 hours since Biden's announcement - the biggest one-day total of any candidate this election cycle. That, along with the nearly US$100 million she inherits from the Biden-Harris fundraising coffers, gives her a firm financial footing for the campaign to come.
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 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.
Harris may also be able to shore up support from black voters, who polls indicate had been drifting away from 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. 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."
Harris's vulnerabilities
For all Harris's potential strengths, there is a reason why some Democrats were initially reluctant to push Biden to step aside, given that his running mate would be the clear successor.
Despite generating Democratic enthusiasm on the subject of abortion, Harris's record as vice-president has been mixed. Early in the administration, she was set the task of addressing the root causes of the migration crisis at the US-Mexico border. A number of missteps and misstatements - including a ham-handed June 2021 interview with NBC News presenter Lester Holt - damaged her standing and opened her to conservative attacks.
Republicans are already condemning her as the president's "border czar", attempting to make her the face of what public opinion polls have found is the Biden administration's unpopular immigration policies.
"Immigration is a soft spot for Democrats in those battleground areas," Israel said. "This is a very salient issue for voters living in those suburbs, fairly or unfairly. They believe that our immigration system is not managed strongly enough."
The Trump campaign will also try to turn the vice-president's prosecutorial background against her - both by highlighting the former president's record of enacting criminal justice reform and by attacking her past prosecutorial and parole decisions.
Another Harris vulnerability is her chequered track record as a candidate. In her 2016 Senate bid, she faced only token opposition from Republicans in deeply Democratic California.
Her one solo run for national office - a bid for the 2020 Democratic presidential nomination - ended in ruins. While she surged early, a combination of fumbled interviews, a lack of clearly defined vision and a poorly managed campaign led her to drop out before even the earliest primary contests.
First impressions
Perhaps the biggest challenge for Harris is that, unlike the president, she is not the incumbent. While she might have the opportunity to distance herself from some of the more unpopular elements of Biden's record, she also does not have the luxury of being a known quantity for voters.
Expect a furious effort by Republicans to paint Harris as too untested and too risky to be president. In effect, Trump now has a greater claim to being the only proven commodity.
The vice-president has a chance, in the days ahead, to make a new first impression with the American public. If she stumbles out of the gate, it could open the door to an extended power struggle that stretches into the Democrat's national convention in late August. They could end up with the party uniting behind a different candidate - or tearing itself apart.
As the past four weeks have shown, fortunes in the White House race can shift quickly and permanently. Harris has punched her ticket to the biggest stage in American politics - now she has to show she can compete.
- This story was first published by the BBC