To borrow from Shakespeare, “Nothing in his life became him like the leaving it.” Joe Biden might have clung on. He could, in his pain and pride, have fought and failed and lost the presidency to a gloating Donald Trump. He could have let his stubborn Irish heart rule his greying Yankee head. But in the end, finally, Biden, old and ill, bravely did the right and honourable thing.

Likening the departing US leader to the Thane of Cawdor is risky, all the same. The latter’s sudden demise opened the way to even greater tragedy, as students of Macbeth and disputed successions know well. Whether Kamala Harris, Biden’s hand-picked heir, can rise from the ashes of a burnt-out presidency is the great question of the hour. Never in modern times has the Oval Office – the world’s biggest, most awesomely powerful job – been so totally up for grabs with a few frenetic weeks to go.

The unforgiving deadline is 19 August, when the Democratic party national convention opens in Chicago. Thursday 22 August is the day the successful nominee must make her or his acceptance speech. After that, there’s no going back, no time for second thoughts. From then until election day on 5 November, it will be all-out war, a fight to the political death with an extremist Republican ticket in arguably the most consequential election since John F Kennedy narrowly defeated Richard Nixon in 1960.

Will Harris get her party’s nod, or face a damaging internal competition? She has big advantages. The vice-president since 2021, she can count on nationwide name recognition – unlike Trump’s far-right white nationalist running mate, the deservedly obscure JD Vance. She has black and Asian-American roots, a potential plus with minority voters. She is the first ever woman to hold the vice-presidency. And at 59, she is definitely not Joe Biden.

Before joining the White House, Harris was a well regarded prosecutor and senator from California. In office, she has earned a reputation, among those who care to look, for championing women’s rights, education and climate action – and for fighting Republican voter-suppression schemes. She is underestimated and mocked by opponents, as vice-presidents typically are. But she has taken hard knocks and kept going. And she could inherit the $100m Biden-Harris campaign war chest.

For the US’s independent and undecided voters, Harris, crucially, is also not Donald Trump. Instead of a grudge match between two grumpy old men, battering each other bloody like cranky Monty Python knights, a fresh choice may soon be on offer – in terms of personality, energy, policy, tone, trustworthiness and moral integrity. It’s a choice that could bring a generational leap. Come January next year, it’s possible a new, younger morning in the US may dawn.

Yet it wouldn’t do to get carried away. No one knows at this point whether one or several of the many other senior Democrats mentioned as possible replacements for Biden will throw their hats into the ring. They include state governors such as Gretchen Whitmer of Michigan, Josh Shapiro of Pennsylvania and Gavin Newsom of California. Then there’s Pete Buttigieg, a former secretary of transportation, Senator Amy Klobuchar of Minnesota and Senator Cory Booker of New Jersey. Some even talk of a Hillary Clinton-Donald Trump rematch, though hopefully they’re just trying to scare people. Yet consider this: what are the chances now of the first ever all-women ticket?

How Biden’s successor will be picked is still unclear. The fact the president has endorsed Harris will carry considerable weight with party moderates. The so-called progressive left – people such as congresswoman Alexandria Ocasio-Cortez – may have different ideas. Having teetered on the brink of pariah-dom, Biden, by stepping aside, has instantly accrued political capital to add to that accumulated during a relatively successful domestic presidency. It will be hard to deny him his choice – and Harris’s potential rivals will be loath to be cast as agents of division.

With his huge haul of committed convention delegates gained during the primary season, could Biden simply deliver the nomination to Harris? No. Now he is not standing, those delegates are free to vote for whomever they choose. If the process unravels uncontrollably into a month-long popularity contest, in which rival candidates compete to make their case, the choice of nominee could come down to a nail-biting, last-minute vote or series of eliminatory votes on the floor of the Chicago convention.

What an extraordinary, riveting, prime-time spectacle that would make. Even if Harris is ultimately chosen by acclamation, what an awe-inspiring exercise in open, real-time democracy it could be. And how utterly dismaying for Trump and his Republican fib-meisters, mudslingers, cable muppets and poll-manglers. The entire nation, indeed the entire democratic world, would fixate on the drama, would be present, in a very real sense, at the creation.

From the crumbling ruins of Biden’s presidency, a new leader may arise, a new champion for American values, the antidote and cure for Trump and Trumpism. The old guy (he is 78) is already spitting blood from his golf cart. Despite his insults, he knows a fresh candidate will be better equipped to call him to account, expose his crimes, skewer his lies, run him ragged. With luck, it will drive him utterly mad.

What happened on Sunday evening was a personal political tragedy. It was a lesson in the impermanence of power. It showed how, in a healthy democracy, seemingly unassailable leaders can be brought low. But it was also a moment of tremendous hope, of renewal, of resurgence. What had begun to look dismayingly inevitable – the return of Trump and his repellent brand of mawkish, divisive, egocentric authoritarianism – is suddenly not inevitable at all. Amid anxiety and sadness, the relief is palpable. The adrenaline flows anew. Now let the battle truly commence.

Thank you, Joe, for finally getting it. You beat the pandemic, you led the recovery, you extended healthcare, created jobs and showed the way on the climate crisis. You were wrong on Gaza and Afghanistan, too cautious on Ukraine. But you were a decent man and, for the most part, a good president for the US and the world. For all this and more you will be gratefully remembered. But leaving is the best thing you ever did.