Chicago, Illinois - Kamala Harris is poised to deliver the biggest speech of her political life on Thursday as she accepts the Democratic presidential nomination in Chicago after a historic turnaround in the 2024 White House race.

The 59-year-old US vice president will focus on joyful "vibes" after electrifying her party in the space of a single heady month since President Joe Biden dropped out of the contest.

Now, Harris will tell her personal story to the American people, using her televised address to the Democratic National Convention to contrast her optimism with the darker tone of Republican Donald Trump.

"It's going to blow the roof off," said Amanda Taylor, a 47-year-old delegate from Missouri.

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Harris and her husband, Second Gentleman Doug Emhoff, also swapped messages on X on Thursday for their 10th wedding anniversary.

"To the best partner I could ask for: Happy anniversary, Dougie," the Vice President wrote, while he said: "Forever to go. Happy anniversary @VP. I love you."

Yet while Democrats' hopes are soaring and as Harris edges ahead in the polls, they know the battle is far from won.

From Barack and Michelle Obama to Bill Clinton, senior figures have warned all week that Harris has a brutal fight on her hands to beat the 78-year-old Trump.

The sheer speed of her astonishing rise to the top of the ticket also means Harris remains an unknown quantity to many US voters.

A trailblazer as the first woman, Black and South Asian vice president in US history – and now bidding to become its first woman president – her role has largely kept her in the background the last four years.

Harris will seek to remedy that in her speech. She will talk about how she was raised by a working mother and knows the challenges facing families hit by inflation, a campaign official said on condition of anonymity.

She will set her confident vision for America's future against what her campaign calls Trump's dark, conservative inspiration for a second term in the Oval Office, the official said.

Speaker after speaker has focused on the idea of freedom during the Democratic convention, as the party targets what it says are Republican plans to further limit abortion and clamp down on democratic institutions.

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On Wednesday, Harris's energetic running mate, Tim Walz, formally accepted the party's nomination, saying: "Kamala Harris is going to stand up and fight for your freedom to live the life that you want to lead."

But Harris has been short on policy announcements since taking over as the Democratic standard-bearer, particularly when it comes to the economy, a key issue in the election.

She has to take advantage of her first major speech in a presidential setting, as "you don't get a second chance to make a first impression," political analyst Larry Sabato told AFP.

"Voters already have the Kamala vibes. Now they need the Kamala agenda," said Sabato, a professor at the University of Virginia. A lack of economic policy "can defeat her faster than the border," he added.

But when it comes to vibes, Democrats are in full-on celebration mode, unrecognizable from the party that was steeped in despair after the 81-year-old Biden's catastrophic debate performance against Trump.

The Obamas raised the roof in Chicago on Tuesday, with the ex-first lady declaring that under Harris, "hope is making a comeback."

On Wednesday, Clinton, television talk show host Oprah Winfrey, and musicians Stevie Wonder and John Legend were the warm-up acts for Walz, the governor of Minnesota.

Biden's farewell address on Monday, when Harris made a surprise appearance on stage to give him a hug, already seems like a distant memory.

If the transition has been head-spinning for Biden and the Democrats, it has completely unsettled Trump.

In a roller-coaster summer, he survived an assassination attempt and then saw what he thought was a certain victory turned on its head by a new and far younger opponent.

Trump will be in Arizona on the Mexican border on Thursday to push Harris' weak spot on the issue of illegal immigration.