Harris's 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's forceful stump speeches have been met by a surge in enthusiasm from voters.
If she wins on November 5, she will be the first Black, South Asian woman elected president.
President Joe Biden threw his support behind Vice-President Kamala Harris after he stepped aside. (AP PHOTO)
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 her Republican rival, former president Donald 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.
Kamala Harris and running mate Tim Walz have been met with a surge of Democratic enthusiasm. (AP PHOTO)
The campaign declined to comment.
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.
Harris is leading in a compilation of national polls by FiveThirtyEight 46.6 per cent to 43.8 per cent for Republican Donald Trump, and has pulled ahead in several public battleground state polls, but the race is tight.
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.
Strategists warn enthusiasm and rising poll numbers are not enough to beat Donald Trump. (AP PHOTO)
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.
It will also include a nod to such allies as unions and lean on Republican voices to persuade conservative voters to abandon Trump.
Former US representative Adam Kinzinger, one of the 10 House Republicans who voted to impeach then-president Trump, said he would speak at the convention on Thursday, one of several Republicans disenchanted with Trump to address the gathering.
