Kamala Harris will be crowned by jubilant Democrats in Chicago this week as the party's standard-bearer against Donald Trump after one of the most head-spinning turnarounds in US political history.

The 59-year-old vice president and her running mate Tim Walz will be the stars of the Democratic National Convention -- with President Joe Biden reduced to a warm-up act after his shock withdrawal from the White House race.

It will be a critical opportunity for Harris, the first Black and South Asian woman to head a major US party ticket, to show Americans her credentials for the Oval Office and ride a wave of excitement into what remains a tight election.

In a remarkable show of unity in front of thousands of delegates, Harris will be backed by three presidents -- 81-year-old Biden plus former presidents Barack Obama and Bill Clinton -- along with former nominee Hillary Clinton.

But there will also be heavy security in Chicago amid plans for mass protests against the Biden-Harris administration's continued support for Israel's devastating war in Gaza.

Just a few weeks ago, the prospect of Harris leading Democrats into the four-day meeting would have seemed far-fetched, but the US political landscape has been completely upended since then.

Instead of the convention being a grim, Biden-led slog towards near-certain defeat after his disastrous debate performance against Trump, the party has dared to hope again.

'All about excitement'

Harris has wiped out Trump's lead in the polls, drawn huge crowds in a blitz of battleground states and raised record funds -- all while bringing what she calls a "joyful" message to the Democratic campaign.

Her televised speech on the final night of the convention on Thursday will now be a chance to define herself to voters and sell her story as America's first Black woman vice president.

"It is all about excitement, that she is the new generation," Casey Burgat of George Washington University told AFP.

"This is about introducing herself as the nominee, what she stands for."

Harris, who won the nomination in a virtual roll call ahead of the convention but will formally accept it this week, will also finally be setting out her policies, having kept things vague until now.

She is expected to focus on cutting high prices that have hit Americans hard -- and which ate away at Biden's popularity -- and to hammer Trump on the issue of abortion.

The Republican former president has been floundering since Harris took over, branding her remarkable rise as a "coup" against Biden and resorting to attacks on her race.

A month ago it was Trump making a triumphant appearance at his party's convention after surviving an assassination attempt, but now the tables are turned as the 78-year-old struggles to deal with a younger candidate.

'Woman of color'

Everything has changed for Biden too, as he prepares to give a valedictory address as a lame-duck president on Monday instead of being the headliner.

Afterwards, he is expected to immediately depart for a holiday in California.

The Democrats have not unveiled their full lineup yet, but US media reported that Hillary Clinton, who lost to Trump in 2016, is also expected to speak on Monday while Obama will address the convention on Tuesday. 

Bill Clinton will be the warm-up act on Wednesday for Minnesota Governor Walz.

The folksy Midwesterner has turned out to be a punchy crowd-pleaser who has led the Democrats' charge on branding Trump and his VP pick J.D. Vance as "weird."

While Harris can expect a further post-convention bump in the polls, the challenge then will be to keep up the momentum in the sprint to election day on November 5.

Polls show she remains vulnerable on key issues such as immigration and the economy.

But her dizzying rise so far shows that she could be the one to break some of the last glass ceilings in US politics.

Political scientist Regina Bateson of the University of Colorado Boulder said "people might have had doubts" about whether a major American party would coalesce around "someone who's a woman of color."

"Actually, she rallied the party around her very quickly, and has been very successful."

By Danny Kemp, AFP