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”.
Mr Walz, 60, said it was “the honor of a lifetime” to join the Democratic ticket.
He is viewed as somebody who can win over rural and working-class voters, particularly in crucial Midwestern US states.
Mr Walz will appear 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.
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.
US media reports indicated Ms Harris had not decided her running mate until Tuesday morning.
Her campaign vetted about a dozen people over the past two weeks.
Announcing the selection, Ms Harris shared details of Mr Walz’s background “both because it’s impressive in its own right, and because you see in no uncertain terms how it informs his record”.
“As a governor, a coach, a teacher, and a veteran, he’s delivered for working families like his own,” she wrote.
“We are going to build a great partnership. We start out as underdogs but I believe together, we can win this election.”
During the vetting process, Mr Walz drew enthusiastic support from pro-labour groups and his party’s progressive wing.
He also gained attention with his description of Mr Trump, Mr Vance and ‘Make America Great Again’ Republicans as “weird”.
That characterisation has since been adopted by several Democrats, including Ms Harris. Mr Trump recently responded, saying “they’re the weird ones”.
Trump’s campaign slammed Mr Walz as a “West Coast wannabe” who “has spent his governorship trying to reshape Minnesota” in the image of a left-wing California.
“If Walz won’t tell voters the truth, we will: just like Kamala Harris, Tim Walz is a dangerously liberal extremist, and the Harris-Walz California dream is every American’s nightmare,” press secretary Karoline Leavitt said.
A source close to the vetting process told CBS, the BBC’s US news partner, that Mr Walz was chosen for his experience and record of accomplishment in an executive role. Other factors included a compelling biography, his ability to be a strong messenger against Trump and Mr Vance, and his strong rapport with the vice-president.
Fellow running-mate contenders as well as outside interest groups have rallied behind his selection.
Gun safety groups hailed his evolution on the issue after the 2018 Parkland, Florida school shooting, while Voters of Tomorrow, a youth voter mobilisation group, said it was “excited to fight for young Americans alongside the Harris-Walz Administration”.
President Joe Biden said in a statement that the two “will be the strongest defenders of our personal freedoms and our democracy”.
“They will ensure that America continues to lead the world and play its role as the indispensable nation.”
As the current two-term governor of Minnesota, Mr Walz has overseen one of the most productive legislative periods in state history.
With unified control of the state legislature, Democrats have enshrined abortion rights, strengthened gun laws, created new paid family and sick leave policies, and implemented a series of other progressive wish-list items.
Republicans criticise him for his response to the Covid-19 pandemic and his slow deployment of the National Guard to deal with violence in the aftermath of George Floyd’s murder in 2020.
Before entering the governor’s office, Mr Walz represented a Republican-leaning district in the US Congress for 12 years.
He won that seat in 2006 – the only Democrat to have won in the mostly rural district over the past three decades.
Mr Walz is a native of rural Nebraska and the son of a school administrator and a stay-at-home mother.
He grew up farming and hunting and served in the Army National Guard for 24 years after joining aged 17.
The young Mr Walz also taught secondary school pupils – first for one year in China, then in Nebraska.
His wife, Gwen Whipple, a fellow teacher, drew him to her native Minnesota, where he taught social studies and geography and coached American football.
He has described his entry into politics as almost accidental. He said he volunteered for John Kerry’s 2004 presidential campaign after being angered by an incident in which two of his pupils were turned away from a George W Bush campaign event.
