Former US Vice-President Joe Biden has joined 19 other hopefuls to declare his presidential bid, putting an end to months of speculation.
In a video announcement, Mr Biden argued that the “core values of the nation… our very democracy, everything that has made America America, is at stake.”
The 76-year-old, who has already run twice for the presidency, enters a crowded race for the 2020 Democratic nomination.
They include Senators Elizabeth Warren, Kamala Harris and Bernie Sanders.
Mr Biden, who served as President Barack Obama’s deputy for two terms, recalled President Donald Trump’s much-criticised remark that there were “very fine people on both sides” of the deadly Charlottesville white nationalist riots of 2017.
“With those words, the president of the United States assigned a moral equivalence between those spreading hate and those with the courage to stand against it,” Mr Biden said.
“I believe history will look back on four years of this president and all he embraces as an aberrant 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.”
A spokesman for Mr Obama said selecting Mr Biden as his running mate “was one of the best decisions he ever made” and the two had “forged a special bond”, but the former president notably stopped short of an endorsement.
A source familiar with Mr Obama’s thinking said the former president believed “a robust primary in 2007 and 2008 not only made him a better general election candidate, but a better president, too.”
Mr Biden, asked by reporters in Delaware why Mr Obama had not endorsed him, replied: “I asked President Obama not to endorse and he doesn’t want to… whoever wins this nomination should win it on their own merits.”
President Trump tweeted: “Welcome to the race Sleepy Joe. I only hope you have the intelligence, long in doubt, to wage a successful primary campaign.”
He is the most experienced of the Democratic candidates. A six-term senator, he ran twice unsuccessfully for president – in 1988 and 2008.
Mr Biden was tipped to run for president again in 2016, the year Mr Trump was elected, but ruled himself out after the death of his 46-year-old son, Beau Biden, from a brain tumour.
He has enjoyed relative popularity among Democrats in recent years, consistently leading every national poll of the Democratic primary tracked by the website RealClearPolitics.