Former US Vice President Joe Biden has formally joined the crowded Democratic presidential contest, betting his working-class appeal and ties to Barack Obama's presidency will help him overcome questions about his place in today's increasingly liberal Democratic Party.
He made his announcement in a video posted on Twitter, declaring: "We are in the battle for the soul of this nation."
"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," Biden said.
"And I cannot stand by and watch that happen."
Thursday's announcement marks the unofficial end of the chaotic early phase of the 2020 presidential season. The field now features at least 20 Democrats jockeying for the chance to take on President Trump next year.
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Trump tweeted: "Welcome to the race Sleepy Joe. I only hope you have the intelligence, long in doubt, to wage a successful primary campaign. It will be nasty - you will be dealing with people who truly have some very sick & demented ideas. But if you make it, I will see you at the Starting Gate!"
Biden, a 76-year-old lifelong politician, becomes an instant frontrunner alongside Vermont Senator Bernie Sanders, who is leading many polls.
Among Democrats, Biden has unmatched international and legislative experience, and he is among the best-known faces in US politics.
But the anti-establishment wave that swept Trump into office has not been kind to either party's statesmen. Biden's team worries about his fundraising ability and his tendency to commit gaffes. His centrist approach in a party moving left on major policy debates raises questions about his appeal.
Four years Trump's senior, Biden would be the oldest person ever elected president should he win. Yet his allies believe the sceptics will ultimately warm to his strong connections to the Obama years.
Biden has said he would campaign as an 'Obama-Biden Democrat', who is as pragmatic as he is progressive. He's aiming to be a conduit between working-class white voters and the younger, more diverse voters who backed Obama.
Obama said through a spokeswoman that selecting Biden as his running mate in 2008 was "one of the best decisions he ever made" and that he frequently relied on his "knowledge, insight and judgment", but didn't explicitly endorse him.
The Republican Party wasted no time seeking to undercut Biden's record, releasing a video on Wednesday questioning economic growth under Obama and Biden while resurrecting conservative arguments against Obama's health care law and a failed investment in green energy company Solyndra.
With a record that stretches half a century, Biden's challenges are easy to find.
Most recently, he struggled to respond to claims that he touched 2014 Nevada lieutenant governor nominee Lucy Flores' shoulders and kissed the back of her head before a fall campaign event. A handful of other women have made similar claims, though none has alleged sexual misconduct.
Biden later pledged in an online video to be "much more mindful" of respecting personal space but joked two days later that he "had permission" to hug a male union leader before addressing the group's national conference.
His first White House bid in 1988 ended after a plagiarism scandal and dropped out of the 2008 race after earning less than 1 percent of the vote in the Iowa caucuses. Later that year, Obama named Biden as his running mate.