Kamala Harris was set to accept the Democratic nomination for vice president Wednesday with an appeal for national unity after what she said had been US President Donald Trump's deadly "failure of leadership."

Accusing Trump of turning "our tragedies into political weapons," Harris will call on Americans to vote for Joe Biden, "a president who will bring all of us together."

Former US President Barack Obama
Former president Barack Obama is the keynote speaker on Day 3 of the Democratic National Convention. Photo: POOL / Alyssa Pointer

"Donald Trump's failure of leadership has cost lives and livelihoods," she said, according to excerpts of the speech released earlier. "We're at an inflection point."

Harris is making history as the first black woman on a major party's White House ticket. If Biden defeats Trump on November 3, she would likewise become the first female vice president in US history.

Kamala Harris
Senator Kamala Harris is to formally accept the nomination to be Democratic presidential candidate Joe Biden's running mate. Photo: GETTY IMAGES / Drew Angerer

America's first black president, Barack Obama, was also set to attack Trump as a failure during his speech on the third night of the all-online Democratic convention.

Obama said that on handing over the White House to Trump in 2017, he thought the Republican "might show some interest in taking the job seriously; that he might come to feel the weight of the office and discover some reverence for the democracy that had been placed in his care."

"But he never did," Obama said, according to excerpts of his speech released early.

Former US VP Joe Biden
Former vice president Joe Biden was formally nominated to be the Democratic presidential candidate. Photo: DEMOCRATIC NATIONAL CONVENTION / -

As a result, Trump has left America's "worst impulses unleashed, our proud reputation around the world badly diminished, and our democratic institutions threatened like never before," Obama said.

Trump responded by telling reporters that Obama had been "a terrible president."

Former US President Bill Clinton
Former president Bill Clinton addressing the Democratic convention. Photo: DEMOCRATIC NATIONAL CONVENTION / -

Former first lady and secretary of state Hillary Clinton, who lost the 2016 presidential election to Trump, will also address the convention, which has been forced due to coronavirus safety measures to be staged entirely online and on television.

US President Donald Trump
President Donald Trump is to deliver his speech accepting the Republican presidential nomination at the White House. Photo: GETTY IMAGES / Sandy Huffaker

Others on the night's program include Massachusetts Senator Elizabeth Warren, who unsuccessfully challenged Biden for the nomination, and Nancy Pelosi, speaker of the House of Representatives.

Former Arizona congresswoman Gabrielle Giffords, who has become a gun control advocate after being shot and severely wounded in a 2011 assassination attempt, is also scheduled to speak along with Emma Gonzalez, a survivor of the Parkland school shooting.

Jill and Joe Biden
"I know that if we entrust this nation to Joe, he will do for your family what he did for ours: bring us together and make us whole," says former US second lady Jill Biden as she addresses the Democratic National Convention after her husband was formally nominated by his party for the presidential election. Photo: DC POOL

Young activists are to address the dangers of climate change and a Hispanic family is to discuss the contributions of immigrants to America.

Much focus will be on Obama. He took a back seat during the Democratic primaries but is now throwing his still considerable sway behind Biden's campaign.

"Tonight, I am asking you to believe in Joe and Kamala's ability to lead this country out of dark times and build it back better," he was set to say in his upcoming speech.

Obama will be speaking two days after his wife, Michelle Obama, opened the convention with a scathing takedown of Trump as "the wrong president for our country."

"He cannot meet this moment," she said.

Tuesday's lineup featured two other former presidents -- 95-year-old Jimmy Carter and Bill Clinton, 74, who called the Trump White House "a storm center."

Biden, 77, the former Delaware senator who served as Obama's vice president for eight years, was officially nominated on Tuesday.

Ahead in the polls, he will deliver his own acceptance speech on Thursday at the conclusion of the four-day convention, which was to have been held in the battleground state of Wisconsin before being shifted to an online format.

Harris, the California senator whom Biden picked to be his vice president, will speak live from Wilmington, Delaware, Biden's hometown and campaign headquarters.

The nomination is the latest in a lifetime of firsts for the 55-year-old daughter of a Jamaican father and Indian mother who were immigrants to the United States.

Harris was the first black attorney general of California, the first woman to hold the post, and the first woman of South Asian heritage to be elected to the US Senate.

Trump has chosen the White House South Lawn as the location for his acceptance speech -- a controversial decision given that presidents are legally required to separate their campaigning from taxpayer-funded governing.

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