Former US Vice-President Joe Biden has declared a presidential bid, putting an end to months of speculation.
In a video announcement, Mr Biden warned that the "core values of the nation... our very democracy, everything that has made America America, is at stake".
The 76-year-old enters a crowded race for the 2020 Democratic nomination.
He is up against 19 other hopefuls, including Senators Elizabeth Warren, Kamala Harris, and Bernie Sanders.
In his announcement, Mr Biden 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."
Mr Biden is the most experienced of the Democratic candidates. A six-term senator, ran twice unsuccessfully for president - in 1988 and 2008 - and later served as President Barack Obama's deputy for two terms.
He was tipped to run 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.
Mr Biden has enjoyed relative popularity among Democrats in recent years. He has consistently led every national poll of the Democratic primary tracked by the website RealClearPolitics. The sheer weight of his experience sets him apart from many of the younger 2020 Democratic hopefuls, and widespread name recognition makes him an immediate front-runner.
Mr Biden is also betting on having the strongest appeal of the Democratic candidates across America's Midwest, where many low-income voters have abandoned the party in recent years in favour of Mr Trump.
The former vice-president's campaign suffered a stumble before it began, when he was forced to address claims he had inappropriately invaded the personal space of women. He apologised, saying: "The boundaries of protecting personal space have been reset," he said. "I understand it and I'll be much more mindful."