Former US Vice President Joe Biden on Thursday formally announced his entry into the race for the Democratic party’s presidential nomination for the 2020 election.
He made his announcement in a video posted on Twitter.
“We are in the battle for the soul of this nation,” Biden said in the video released online, calling on voters to deny Republican US President Donald Trump a second term in office.
In the video announcement, Mr Biden argued that the "core values of the nation... our very democracy, everything that has made America America, is at stake".
"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 76-year-old, who has already run twice for the presidency, enters a crowded race for the 2020 Democratic nomination.
He is up against 19 other hopefuls. They include Senators Elizabeth Warren, Kamala Harris and Bernie Sanders.
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."
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 tumor.
He has enjoyed relative popularity among Democrats in recent years, consistently leading every national poll of the Democratic primary tracked by the website RealClearPolitics.
But the former vice-president has been forced to address claims he inappropriately invaded the personal space of women.
He apologized, saying: "The boundaries of protecting personal space have been reset. I understand it and I'll be much more mindful."
Joe Biden enters the Democratic presidential contest as a front-runner, with huge name recognition and the potential to raise vast amounts of campaign donations.
Of course, so did Hillary Clinton in 2015 - and we all know how that turned out.
The former vice-president has a lot going for him. He also has a lot going against him. The durability of his campaign is one of the big questions hovering over the Democratic race.