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Democratic presidential candidate Joe Biden is holding his first campaign event with Kamala Harris, a day after unveiling her as his running mate.
At a high school in Mr Biden’s home state of Delaware, he said “this is a serious moment for our nation” ahead of November’s election.
Later on Wednesday, they will hold a virtual town hall from the hotel where Mr Biden launched his 1972 senate bid.
Mr Biden will face President Donald Trump in the election on 3 November.
Ms Harris, a California senator, is the first black woman and South Asian American to serve in the role.
Wednesday’s event was not open to the public, with Mr Biden citing coronavirus prevention needs.
Shortly before the event, Mr Trump goaded Mr Biden for remaining at home for much of the campaign so far amid the pandemic lockdown.
The Republican president asked a group of teachers at a White House event whether it was healthy for students to be learning in isolation at home.
He then asked: “So if you’re a presidential candidate and you’re sitting in a basement and you’re looking at a computer, that’s not a good thing?”
He also tweeted an attack on the Biden-Harris ticket, saying they would put Senator Cory Booker, who is black, in charge of low-income housing in the suburbs. Critics said the tweet was racist.
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Earlier in the day, Mr Biden, 77, tweeted that his administration would “inherit multiple crises, a nation divided, and a world in disarray.
“We won’t have a minute to waste. That’s exactly why I picked her: She’s ready to lead on day one.”
Ms Harris, 55, tweeted a video saying “I’m ready to get to work”.