Mr. Biden responded by terming Mr. Sanders “the most powerful voice for a fair and more just America.”
Former Vice President Joe Biden, the likely Democratic presidential nominee, was formally endorsed on Monday by longtime rival Bernie Sanders, a move that may help unify the Democratic Party ahead of a general-election matchup against Republican President Donald Trump.
Mr. Sanders, the progressive firebrand who last week suspended his presidential campaign, joined Biden in an online campaign event, citing the coronavirus outbreak as a reason for the party to come together.
That made for a stark contrast to 2016, when Mr. Sanders waited until the eve of the Democratic National Convention in the summer to support the then-nominee, Hillary Clinton.
Before the pandemic brought the Democratic primary contest to a virtual halt, Mr. Biden had built up a commanding delegate edge over Sanders, leading the Vermont senator to declare last week he saw no viable path to the nomination.
Even so, it remained unclear how soon Mr. Sanders, who is especially popular with liberal young voters, would rally behind Mr. Biden, whose support comes chiefly from the party’s older moderates.
In the online event, Mr. Sanders called on Democrats, independents and Republicans “to come together in this campaign to support (Mr. Biden’s) candidacy, which I endorse, to make certain that we defeat somebody who I believe… is the most dangerous president in the modern history of this country.”
Mr. Biden responded by terming Mr. Sanders “the most powerful voice for a fair and more just America.
With social distancing in widespread effect, Mr. Biden has been forced to move the bulk of his campaign to online events and has watched as the global pandemic had relegated the 2020 presidential race to the back burner.
The Sanders endorsement gives him a jolt of energy and attention at time when his campaign could use it.
Mr. Biden said he and Sanders would develop six policy working groups on issues including the economy, education, criminal justice, immigration and climate change to develop solutions to problems in the United States.
“I’m going to need you,” Mr. Biden said to Mr. Sanders, “not just to win the campaign, but to govern.”
Mr. Biden and his campaign had been reaching out to Sanders and his aides for weeks, even before he dropped out. Mr. Biden and Mr. Sanders spoke personally, and Mr. Biden avoided suggesting that Sanders drop out, according to a person familiar with that conversation.
Mr. Biden’s campaign also had some lower-level talks with the Sanders campaign that included efforts to make an exit appear constructive for Sanders and making clear that there was room for ideas in Biden’s general election campaign, according to a person familiar with the matter.
This included conversation between a top Biden aide, Anita Dunn, and a counterpart with Sanders, Jeff Weaver. Neither could be reached for comment.
The person said that Mr. Biden would not be likely to adopt a wide swath of Sanders policies wholesale, such as Medicare for All, with which Mr. Biden does not agree.
Yet in recent days Mr. Biden did endorse expanding access to Medicare, forgiving some student debt and making public colleges free for families making $125,000 or less, positions closer to ones embraced by Mr. Sanders.
Beyond courting Mr. Sanders, Bidens campaign had begun to reach out to progressive advocacy groups centered around issues such as climate change and racial justice in a bid to the heal the longtime tension between the moderate and progressive camps within the party.
Mr. Trump has tried to take advantage of those fissures, working on social media to try to keep Sanders supporters away from Mr. Biden, suggesting that Mr. Sanders had been forced out of the race by the Democratic establishment.