The pair held their first campaign event together, a day after Mr. Biden unveiled Ms. Harris as his number two.
President Trump hit back, saying Ms. Harris had “dropped like a rock” in her own presidential bid.
Mr. Biden will face President Trump, a Republican, in November’s election.
What did Joe Biden and Kamala Harris say?
Mr. Biden noted that Ms. Harris, a US senator from California, was the first woman of colour to serve as a presidential running mate for a major US party.
Mr Biden said: “The choice we make this November is going to decide the future of America for a very, very long time.”
He continued: “Donald Trump has already started his attacks, calling Kamala, quote, nasty, whining about how she is, quote, mean to his appointees.
“It’s not a surprise because whining is what Donald Trump does best, better than any president in American history.
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