Harris’ own presidential ambitions were always clear but had been undermined by her own shaky 2020 campaign and bumpy vice-presidential term.

Since being thrust to the top of the ticket, she has tightened the race against Republican Donald Trump.

Harris’ forceful stump speeches have been met by a surge in enthusiasm from voters.

In her speech, aides and advisers said Harris, 59, planned to tell the story of who she was – the daughter of a Jamaican father and Indian mother – and of her plans to help the middle class with cuts in prices on groceries and housing and in taxes and to advance personal freedoms including abortion rights.

They said she would also deliver a robust denunciation of Trump.

“There is a guy who wants to divide us and she will make the case that we simply cannot let that happen, that this is America and everybody can rise together,” Cedric Richmond, campaign co-chair and longtime adviser to Harris, told Reuters.

Convention delegates got a preview when Harris unexpectedly walked out on stage to the tune of Beyonce’s Freedom on the convention’s first night.

“This November we will come together and declare with one voice, as one people – we are moving forward,” she said.

Whether Beyonce will appear on stage on Thursday is a matter of much speculation and debate in convention hallways and meeting places.

Harris has raised a record-breaking $US500 million ($A743 million) in a month, and narrowed the gap or taken the lead against Trump in many polls of battleground states.

But the US vice-president has yet to articulate much of her vision for the country.

Enthusiasm and rising poll numbers are not enough to beat Trump in 11 weeks, strategists warned.

On Monday, the founder of the main outside spending group backing Harris’s presidential bid said internal polling was less “rosy” than public polls suggested and warned Democrats faced much closer races in key states.

Harris has yet to articulate much of her vision for the country, and Republicans say Democrats have spent much of their time attacking Trump rather than explaining how they would govern.

“In many ways this race has not yet even begun but when it does, I hope, speaking not as a Republican but as an American, it’s about competing policy visions,” Vivek Ramaswamy, a former Republican presidential candidate, said at a news conference.

Harris has spent weeks on the speech, making changes to drafts from lead speechwriter Adam Frankel, including during campaign trips on Air Force Two.

Her focus on what she has called “price gouging” has drawn criticism from Republicans who say Democrats should trust free markets rather than set prices.

“She will highlight her message on prices because that is resonating really well with people, despite Republican attacks,” an aide to the vice-president said.

The speech will include elements of foreign policy along with stories of women affected by abortion bans and other curbs on reproductive rights, the aides and advisers said.

Trump has spent the week campaigning in the handful of battleground states that will decide the election.

He visited the US-Mexico border in Arizona on Thursday afternoon, where he will likely accuse Harris and her running mate Tim Walz of not doing enough to prevent illegal immigration.

“If these two people take it over, this country is finished,” he said on Fox News on Thursday morning.

“Open borders, no drilling, our country will die.”