US Vice-President Kamala Harris will make the most important speech of her career on Friday (AEST) when she formally accepts her party’s presidential nomination to wrap up the Democratic convention.

It comes a day after her running mate Tim Walz whipped up a frenzy with his footy coach “pep talk” and talk show queen Oprah Winfrey made a surprise appearance on the third night. 

Aids and advisers say Harris, 59, plans to tell the story of who she is — the daughter of a Jamaican father and Indian mother — in what could be a make-or-break moment for her campaign.

The Democrat candidate is expected to outline her plans, if she is elected US president, to help America’s middle class.

Her promises include to bring down rising grocery and housing prices, cut taxes for 100 million middle and low-income earners and advance personal freedoms including abortion rights.

Harris’s advisers 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.

President Joe Biden passed the torch to Kamala Harris on Tuesday (AEST). Photo: Getty

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 kerbs 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.

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.

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

Trump blows dog whistle

On the same day, meanwhile, Trump is due to speak in Arizona and focus on the issue of immigration as he visits the US-Mexico border.

Trump has been the butt of numerous Democrat zingers at the National Convention, including from Barack and Michelle Obama and Bill and Hillary Clinton.

As the attacks continued, Trump appeared to want to push back against advice from within the Republican party to stay on topic and avoid personal hits on his opponents.

In recent days, Trump has resorted back to one of his favoured dog-whistle tactics of racial innuendo to appeal to his supporters.

After the Obamas criticised Trump at the DNC on Tuesday (US time), Trump asked the crowd if they’d seen the speech by the man he continues to call “Barack Hussein Obama.”

“Did you see Barack Hussein Obama last night,” said Trump.

“He was taking shots at your president. And so was Michelle.”

He asked the audience whether they thought he should “stick to policy” given the sledges against him.

“You know, they always say, ‘Sir, please stick to policy. Don’t get personal.’

“And yet they’re getting personal all night long, these people,” he said, asking: “Do I still have to stick to policy?”

Trump also took to social media to criticise Democratic Pennsylvania Governor Josh Shapiro as a “highly overrated Jewish Governor”.

“I have done more for Israel than any President…Shapiro has done nothing for Israel, and never will,” he wrote.

A tight race

The biggest speech of Harris’s career comes just a month after the party forced President Joe Biden to exit the race.

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 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, southern Asian woman elected president.

Harris has raised a record-breaking $US500 million ($743 million) in a month, and narrowed the gap or taken the lead against Trump in many polls of battleground states.

But she 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 Trump, and has pulled ahead in some 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.

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, one of several Republicans disenchanted with Trump to address the gathering.

-with AAP