Joe Biden 2020: Former vice-president enters US election race in 'battle for soul of this nation'
The 76-year-old lifelong politician becomes an instant front-runner alongside Bernie Sanders, who is leading many polls.
In his campaign video, released on Thursday morning, Mr Biden warned that handing Mr Trump a further four years in the White House would "forever and fundamentally alter the character of this nation".
He also attacked the president for his response to a white supremacist rally in Charlottesville, Virginia, in 2017, accusing Mr Trump of having "assigned moral equivalence between those spreading hate and those with the courage to stand against it".
“In that moment, I knew that the threat to this nation was unlike any I had ever seen in my lifetime,” he added.
“We are in the battle for the soul of this nation.
"I believe history will look back on four years of this president, and all he embraces, as an abhorrent moment in time, but if we give Donald Trump eight years in the White House, he will forever and fundamentally alter the character of this nation, who we are. And I cannot stand by and watch that happen.
"The core values of this nation, our standing in the world, our very democracy; everything that has made America America is at stake."
The announcement, made in a video posted on Twitter, marks the unofficial end of the chaotic early phase of the 2020 presidential season.
At least 20 Democrats have put their names in the hat for the chance to take on the incumbent Republican president next year, while several lesser-known candidates may still join the race.
"Amtrak Joe" – a moniker he earned for commuting daily to the US capital by rail during his time as senator – will hope to capitalise on his down-to-earth manner and blue-collar upbringing if he is to win the Democrat nomination.
He remains one of the most recognisable names in US politics, but as an older white man who spent a half-century in Washington, it is unclear if he will be embraced by an increasingly liberal Democratic Party.
He has also been dogged by questions about his past, including recent claims he touched women in an overly familiar manner without their consent. Mr Biden has pledged to be "much more mindful" of respecting personal space.
Four years Mr Trump's senior, Mr Biden – who has said he would campaign as an "Obama-Biden Democrat" – would be the oldest person ever elected president should he win.
The former vice president will be in the Pennsylvania, a state that swung to Mr Trump in 2016 after voting for Democratic presidential candidates for decades, three times within the opening weeks of his campaign, the Associated Press reported.
He will be in Philadelphia on Thursday evening headlining a fundraiser at the home of David L Cohen, executive senior vice president of Comcast, where he hopes to raise $500,000 (£388,000), and will return to the city in the next two weeks for a major rally.
The Republican Party responded to rumours of an imminent announcement by Mr Biden with a video released on Wednesday questioning economic growth during the Obama presidency and resurrecting arguments against the former president's health care law.
The video ends with the words, "Joe Biden: Backwards, not forwards."