Barack Obama, America's first black president, is the keynote speaker at the Democratic convention Wednesday as Kamala Harris, the first black woman on a major party ticket, formally accepts the nomination to be Joe Biden's running mate.

Former first lady Hillary Clinton, who lost the 2016 presidential election to Republican Donald Trump, Biden's November opponent, is also featured on the program on Day 3 of a convention being held almost entirely online for the first time.

Others on the bill include Massachusetts Senator Elizabeth Warren, Nancy Pelosi, speaker of the House of Representatives, and former Arizona congresswoman Gabrielle Giffords, who has become a gun control advocate after being shot and severely wounded in a 2011 assassination attempt.

The highlights of the evening are expected to be Obama's speech and the formal nomination of Harris to be the Democratic vice presidential candidate.

Obama took a back seat during the hotly contested Democratic primaries but is expected to play a major role in the campaign to get Biden elected and oust Trump from the White House.

In a fund-raising message ahead of his speech, Obama said voters faced a choice between the "politics of cynicism" and "politics of hope."

"The Republican Party made its choice long ago, and we're seeing the consequences of it every day," Obama said.

"They have no answers for the COVID-19 pandemic that has stolen the lives of more than 170,000 Americans and shuttered countless businesses -- just excuses and blame.

"Joe understands that when times get tough, we don't give up -- we get up," Obama said.

"And so it's time for us all to get up, right now, and stand with Joe in this fight for the soul of America."

'There's only chaos' 

Obama will be speaking two days after his wife, Michelle Obama, opened the convention with a scathing takedown of Trump, painting him as a man who lacks the competence, character and decency for the job.

"Donald Trump is the wrong president for our country," she said. "He has had more than enough time to prove that he can do the job, but he is clearly in over his head.

"He cannot meet this moment."

Tuesday's lineup featured 95-year-old president Jimmy Carter, who served one term from 1977, and 1990s commander-in-chief Bill Clinton, who warned that the Trump White House is swirling with chaos instead of the competence necessary to address the nation's crises.

"At a time like this, the Oval Office should be a command center. Instead, it's a storm center. There's only chaos," Clinton said.

Biden, 77, the former Delaware senator who served as Obama's vice president for eight years, was officially nominated on Tuesday to take on Trump in the November 3 election.

Biden, who is leading in the polls, will deliver an acceptance speech on Thursday at the conclusion of the four-day convention, which was to have been held in the battleground state of Wisconsin but was shifted to an online format because of the coronavirus pandemic.

California Senator Harris, Biden's choice for vice president, is to formally accept her nomination on Wednesday.

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.

She is the first black woman to feature on the ticket of a major US political party and is seeking to become the first female vice president of the United States.

The Republican Party is to hold its virtual convention next week and nominate Trump to serve four more years.

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.