Joe Biden has turned the US election on its head. After fiercely insisting for weeks that he would remain the Democratic nominee for president, he has bowed to pressure and dropped out of the race.

Here's what it means for Vice-President Kamala Harris, for the Democrats more broadly and for Donald Trump.

The prospects for Kamala Harris being the Democratic nominee have received a big boost with Joe Biden’s endorsement.

He gave her his full backing, calling his decision to make her vice-president four years ago the best he ever made.

She responded by saying she was honoured to have his endorsement and would do everything possible to win the nomination.

It is possible that most Democrats will follow the president’s lead and fall in line behind the vice-president to avoid ongoing uncertainty less than a month from the Democratic convention.

There are practical and political reasons for doing so.

She is next in the constitutional line of succession. The optics of passing over the first black woman on a presidential ticket would be terrible for the party. She would also immediately have access to the roughly $100 million in funds the campaign has raised so far.

But there are also risks. Public opinion surveys show Harris’ approval ratings are about as low as his. And in head-to-head matchups against Donald Trump, she fares roughly the same as Biden.

Second is that Harris has had a sometimes rocky time as vice-president. Early in the administration, she was given the task of addressing the root causes of the migration crisis at the US-Mexico border.

That’s a daunting challenge, and a number of missteps and misstatements opened her up to criticism. She’s also been the administration’s point person on abortion rights, which has been a topic she has much more effectively handled. But those first impressions have stuck.

Finally, and perhaps most importantly, Harris has already run for national office - her 2020 bid for the Democratic presidential nomination - and stumbled badly.

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 primaries.

Opting for Harris is a risk for Democrats, but at this point there are no safe options. And the stakes – a possible Donald Trump victory – are as high as they get.

Over the past half-century, political conventions have been transformed into somewhat boring affairs. With every minute carefully scripted for television, they’ve become extended multi-day commercials for the presidential nominee.

Last week’s Republican convention was certainly that way - even with Donald Trump’s overly long, sometimes rambling nomination acceptance speech.

Next month’s Democratic convention in Chicago is shaping up to be very, very different. Whatever script the party and the Biden campaign had been working on just got thrown out the window. Even if the party falls in line behind Harris, it will be difficult to plan - and control - how things unfold on the convention floor.

And if Harris doesn’t succeed in uniting the party, the convention could turn into a political free-for-all, with various candidates vying for the nomination before the cameras and behind closed doors.

It could make for gripping political theatre, live and unpredictable, in a way the American public has never before witnessed.

This year’s Republican convention was a carefully calibrated machine, promoting the party’s most popular agenda items and focusing criticism on one man, President Joe Biden.

It turns out, the Republicans were targeting the wrong guy.

With the news of Biden’s abandoning of his re-election campaign, the Republican game plan spearheaded by Donald Trump has been turned on its head.

The Republicans spent an entire week of carefully scripted events focusing on the wrong weaknesses of the Democrat opposing them.

The campaign had highlighted their candidate’s strength and vitality by giving him a raucous entrance, preceded by appearances by former wrestler Hulk Hogan and Ultimate Fighting Championship impresario Dana White, as well as a performance by Kid Rock.

The attempts at contrast with Mr Biden’s perceived frailty - and the strategy to peel away younger male voters - were obvious.

But in any scenario now, the Democratic nominee is going to be someone much younger than the president.

A strategy of strong vs frail against Vice-President Kamala Harris or one of the more youthful Democratic governors who are mentioned as possible Biden successors just won’t pack the same punch.

If Harris is the nominee, expect the Republicans to try to tie her to the perceived failings of the current administration. For months they have called her the "border czar".

Although the former prosecutor is by no means from the progressive wing of the party, previous Republican attacks on her suggest they may also paint her as "radical left".

No matter who the nominee is, the Republicans are sure to blame the Democrats for covering up Biden’s age-related weaknesses - and putting the nation at risk.

At this point, everyone is flying blind with just a few months until the first presidential ballots are cast.