The two leaders were holding a virtual town hall meeting on the impact of the Corona epidemic on women from their homes.
Clinton gave his support in a video calling for Biden on Tuesday evening Danish time.
"You won the majority of the vote", Biden said during a livestream conversation with Clinton, his party's 2016 nominee.
Clinton told the former vice president: "I am thrilled to be part of your campaign to not only endorse you but to help highlight a lot of the issues that are at stake in this presidential election".
The endorsement marks the latest in a series of high-profile Democrats lining up behind Biden ahead of what is expected to be a tight November 3 election rematch between their party and Republican President Donald Trump.
Hillary made the declaration during a live event with Biden on Tuesday.
There is a small downward movement in support among African Americans for Biden, but it is unclear whether it is statistically significant. "He has gained ground among voters under 35, a group that had been more likely in the primaries to support Vermont Sen".
Clinton sat out the primary fight, telling interviewers repeatedly that she was paying attention to the nominating process from afar.
"I think the way in which some of the states acted was just, anyway", he said, before praising how Clinton would have handled the crisis. "That's the only thing that really matters at the end of the day".
- I wish you were president right now, she adds. "He knows what needs to be done, he can fix the damage that he would be inheriting".
Biden suggested in his 2017 book, "Promise Me, Dad", that Obama favored Clinton's 2016 presidential bid over the possibility of Biden running.
The decision not to run back then has weighed on Biden, though.
"Mark my words, I presume he's gonna attempt to kick the election back, develop some rationale why it cannot be held", Biden said through an internet fundraiser.
Although he has endorsed Biden, Sanders stays on the ballot in the intra-party elections and wants to use his showing as a bargaining chip in drafting the party's election manifesto.
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