Former US vice-president Joe Biden was instantly installed as the Democrats' front runner after launching a bid yesterday for the White House.
Mr Biden announced the third presidential bid of his career by video on YouTube, drawing a stark contrast between himself and president Donald Trump in a contest he said was a fight for the future of American democracy.
"We are in the battle for the soul of this nation," he 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 let that happen."
Mr Trump responded with a tweet slamming Mr Biden's "intelligence" and vowing to meet him "at the Starting Gate" if the Democrat wins his party's "nasty" nomination fight.
Mr Biden (76) had been wrestling for months over whether to run. His candidacy will face numerous questions, including whether he is too old and too centrist for a Democratic Party yearning for fresh faces and increasingly propelled by its more vocal liberal wing.
Mr Biden starts as leader of the pack in opinion polls of a Democratic field of 20 contenders seeking a chance to challenge Mr Trump, the likely Republican nominee, in November next year.
Critics say his standing in polls is largely a function of name recognition for the former senator from Delaware, whose more than four decades in public service includes eight years as Barack Obama's number two in the White House.
Mr Obama's spokeswoman, Katie Hill, said Mr Obama had long said that selecting Mr Biden to be his running mate in 2008 was one of the best decisions he ever made.
She said Mr Obama relied on Mr Biden's insight, knowledge and judgment throughout his presidency.
Mr Biden will travel across the US in the coming weeks to detail his plans to rebuild the middle class, kicking- off his tour in Pittsburgh on Monday, his campaign said.
On May 18, he will hold a rally in Philadelphia to "lay out his vision for unifying America with respected leadership on the world stage and dignified leadership at home", it said.
As speculation about his bid mounted, Mr Biden faced new questions about his propensity for touching and kissing strangers at political events, with several women coming forward to say he had made them feel uncomfortable.
Mr Biden struggled in his response to the concerns, at times joking about his behaviour.
Ultimately, he apologised and said he recognised standards for personal conduct had evolved in the wake of the #MeToo movement.
In his announcement yesterday, Mr Biden held up the example of the August 2017 attack at a white nationalist rally in Charlottesville that killed a female counter-protester as a defining moment for the nation.
"It was there we saw Ku Klux Klansmen and white supremacists and neo-Nazis come out in the open, bearing the fangs of racism," Mr Biden said, criticising Mr Trump's remarks at the time that there were "very fine people on both sides".
There was no moral equivalence between racists and those fighting such inequality, he said.
"In that moment, I knew the threat to this nation was unlike any I had ever seen in my lifetime," said Mr Biden.
His candidacy will offer early hints about whether the Democrats are more interested in finding a centrist who can win over the white working-class voters who backed Mr Trump in 2016, or someone who can fire up the party's progressive wing, such as Kamala Harris, Bernie Sanders or Elizabeth Warren.
Mr Biden's long history in the Senate, where he was a leading voice on foreign policy, will give liberal activists plenty to criticise.
As Senate judiciary committee chairman, he angered women's rights activists with his handling of sexual harassment allegations against Clarence Thomas during the justice's 1991 Supreme Court confirmation hearings.
He has also been censured over his ties to the financial industry. Mr Biden has been one of the party's more aggressive Trump critics. Last year, he said he would have "beat the hell" out of the president if the two were in high school because of the way he has talked about women.
That prompted Mr Trump to call him "Crazy Joe Biden" and to claim on Twitter that Mr Biden would "go down fast and hard, crying all the way" if they fought. Mr Biden later lamented the exchange, saying: "I shouldn't get down in the mosh pit with this guy."