Democratic US presidential candidate Joe Biden and new running mate Kamala Harris have made their first joint appearance on the campaign trail, with Mr Biden saying Ms Harris was the right choice to help him kick Donald Trump out of the White House.

In a high school gymnasium on Wednesday in his hometown of Wilmington, Delaware, Mr Biden praised Ms Harris as an experienced and accomplished governing partner who would help him deliver on his campaign agenda.

“I have no doubt that I picked the right person to join me as the next vice president of the United States of America, and that is Senator Kamala Harris,” Mr Biden said in introducing Ms Harris, who sat behind him on stage to watch Mr Biden’s speech.

“She’s ready to do this job on day one. We’re both ready to get to work rebuilding this nation,” he said.

“This morning, all across the nation, little girls woke up, especially little Black and brown girls, who so often feel overlooked and undervalued in their communities.”

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The joint appearance came just days before Mr Biden will formally accept the Democratic presidential nomination at next week’s party convention, which will take place largely as a virtual event because of the coronavirus pandemic.

Tweet from @JoeBiden

The Republican convention, where President Donald Trump is set to be nominated to seek a second four-year term, follows a week later and kicks off a 10-week sprint to the November 3 election.

Ms Harris, the 55-year-old US senator from California, was announced as Mr Biden’s choice on Wednesday after a selection process that drew extra scrutiny thanks to Mr Biden’s age.

The 77-year-old former vice president would be the oldest president ever if he wins, raising speculation that he would not seek re-election in 2024.

Ms Harris, the first black woman and the first Asian American to appear on a major-party US presidential ticket, is the daughter of two immigrants, her mother from India and her father from Jamaica.

Wednesday’s event in Mr Biden’s East Coast hometown took place on the third anniversary of the “Unite the Right” rally in Charlottesville, Virginia, where violent clashes erupted as white supremacists and neo-Nazis descended on the town.

Tweet from @KamalaHarris

Mr Biden has said Mr Trump’s comment after the clashes that there were “very fine people on both sides” – which drew widespread criticism – convinced him to run for president.

In recent months, as unrest has convulsed many US cities following the May police killing of George Floyd, a Black man in Minneapolis, Ms Harris has become a prominent voice calling for change.

She has marched alongside protesters and pushed legislation to reform policing practices.

A crowd of about 75 people gathered outside in a light rain before the event to get a glimpse of Mr Biden and Ms Harris, holding signs and snapping pictures.

Most were supporters and some chanted Ms Harris’ name, although some were Mr Biden opponents.

In choosing Ms Harris, Mr Biden selected a former rival for the nomination whose most memorable campaign moment came during a televised debate when she criticised his past position on using busing to integrate schools.

But Ms Harris was regarded as a relatively safe choice. She is a more dynamic campaigner than Mr Biden and will be relied upon to help energise black voters, who represent a crucial constituency for Biden in election battleground states.

-AAP