Joe Biden has chosen Senator Kamala Harris as his running mate, making her the first black woman on a major party presidential ticket.
Mr Biden made the announcement on Twitter, calling it a "great honour" to name Ms Harris as his number two.
He said the California senator was "a fearless fighter for the little guy, and one of the country’s finest public servants", also referencing her work alongside his son, Beau, when she was the state's attorney general.
"Back when Kamala was Attorney General, she worked closely with Beau," he added.
"I watched as they took on the big banks, lifted up working people, and protected women and kids from abuse. I was proud then, and I'm proud now to have her as my partner in this campaign."
Ms Harris, a 55-year-old first-term senator and former rival of Mr Biden's from the Democratic primary, is one of the party’s most prominent figures.
She quickly became a top contender for the number two spot after her own White House campaign ended.
She joins Mr Biden in the 2020 race at a time of unprecedented national crisis. The coronavirus pandemic has claimed the lives of more than 150,000 people in the US, far more than the toll experienced in other countries.
Business closures and disruptions resulting from the pandemic have caused an economic collapse.
Meanwhile unrest has emerged across the country as Americans protest racism and police brutality.
Donald Trump’s uneven handling of the crises has given Mr Biden an opening, and he enters the autumn campaign in strong position against the president.
In adding Ms Harris to the ticket, he can point to her relatively centrist record on issues such as healthcare and her background in law enforcement in the nation’s largest state.
She was California attorney general and district attorney in San Francisco, and her record was heavily scrutinised during the Democratic primary.
Some liberals and younger black voters saw her as out of step on issues of systemic racism in the legal system and police brutality.
Read More
She tried to strike a balance on these issues, declaring herself a “progressive prosecutor” who backs law enforcement reforms.
Mr Biden, who spent eight years as president Barack Obama’s vice president, has spent months weighing who would fill that same role in his White House.
He pledged in March to select a woman as his vice president.
Mr Biden considered Massachusetts Sen Elizabeth Warren, a leading progressive, Florida Rep Val Demings, whose impeachment prosecution of Mr Trump won plaudits, California Rep Karen Bass, who leads the Congressional Black Caucus, former Obama national security adviser Susan Rice and Atlanta Mayor Keisha Lance Bottoms, whose passionate response to unrest in her city gained national attention.
A woman has never served as president or vice president in the United States.
Two women have previously been nominated as running mates on major party tickets: Democrat Geraldine Ferraro in 1984 and Republican Sarah Palin in 2008. Their party lost in the general election.
The vice presidential pick carries increased significance this year. If elected, Mr Biden would be 78 when he is inaugurated in January, the oldest man to ever assume the presidency.
He has spoken of himself as a transitional figure and has not fully committed to seeking a second term in 2024. If he declines to do so, his running mate would likely become a front-runner for the nomination that year.
Additional reporting by the Associated Press