Kamala Harris has chosen Minnesota Governor Tim Walz, a former school teacher and member of Congress, as her vice-presidential nominee.
Ms Harris wrote on social media that he stood out from the competition because “his convictions on fighting for middle class families run deep”.
But the Trump campaign said Mr Walz, 60, was a “dangerously liberal extremist”.
He is viewed as somebody who can win over rural and working-class voters, particularly in crucial Midwestern US states.
Mr Walz has since appeared with Ms Harris later on Tuesday at a campaign event in Philadelphia, Pennsylvania, before the pair begin a five-day tour of other key battleground states.
They will also speak at the Democratic National Convention later this month, from 19 to 22 August in Chicago, when the Harris-Walz ticket will be made formal.
He said earlier it was “the honor of a lifetime” to join the Democratic ticket.
Ms Harris was officially selected as the party’s presidential nominee by a virtual roll-call vote that ended on Monday.
They will face Donald Trump and his running mate, Ohio Senator JD Vance, in the general election on 5 November.
