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Bernie Sanders offered his full-throated endorsement for Joe Biden on Monday, handing the apparent Democratic presidential nominee a crucial boost among the party's progressive wing heading into the general election campaign.

"Today I am asking all Americans, I'm asking every Democrat, I'm asking every independent, I'm asking a lot of Republicans to come together in this campaign to support your candidacy, which I endorse," Sanders said, "to make certain that we defeat somebody who I believe, and I'm speaking just for myself, now is the most dangerous president in the modern history of this country."

Sanders, an independent senator from Vermont, made the endorsement during a surprise appearance on a Biden virtual campaign event after tweeting about the need to unite against President Donald Trump.

"We've got to make Trump a one-term president, and we need you in the White House," Sanders said during the livestream.

"I will do all that I can to see that that happens," Sanders said. "It's imperative that all of us work together to do what has to be done. Not only in this moment, but beyond this moment in the future of this country."

Biden said Sanders' endorsement "means a great deal to me."

"We're awfully close on a whole bunch of issues," Biden said, adding that "if I'm the Democratic nominee, which it looks like now you just made me, I'm going to need you, not just to win the campaign but to govern."

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"I promise I won't let you down," Biden added moments later.

Sanders' endorsement came five days after he ended his own presidential bid. Although Sanders said then that his decision was "difficult and painful," he made it clear that he intended to support Biden, calling him "a very decent man who I will work with to move our progressive ideas forward."

By contrast, in 2016, it took Sanders 36 days to formally endorse Hillary Clinton after she became the party's presumptive nominee.

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Biden has not yet become his party's presumptive nominee. For that to occur, he must win the necessary number of pledged delegates that allows him to formally clinch the nomination. While Biden is the clear leader in the total delegate count, he has not yet reached the needed 1,991.

Sanders, meanwhile, added that he was "very pleased" that his staff and Biden's staff had "been working together over the last several weeks" on policy and to create task forces related to "some of the most important issues facing this country," including criminal justice, health care and climate change.

Sanders said he hoped the task forces would help the two teams "work out real solutions to these very, very important problems," even though he and Biden "have our differences."

NBC News reported last week that Sanders' team had been coordinating extensively with Biden's camp since early March.

Sanders also appeared to talk directly to his own supporters at one point, urging them to move "forward together" and support Biden. He also expressed confidence that Biden was an "inclusive" candidate who wants to "bring people in" to his campaign, including those he disagrees with.

Biden said later that his campaign has an "enormous responsibility" to fully win over Sanders' supporters.

Following their initial exchange on the livestream, the pair engaged in a question-and-answer session, with Biden responding to Sanders' questions about policy.

The discussion covered several issues, including Biden's recently released free college plan — a new policy position borrowed from Sanders and Sen. Elizabeth Warren, D-Mass., that was clearly aimed at courting progressive voters.

The Trump campaign ripped Biden moments after Sanders' endorsement, saying the former vice president "had to adopt most of Bernie's agenda to be successful in the Democrat primaries."

"One thing that is missing is enthusiasm, however, as almost no one is excited about a Biden candidacy," Trump's campaign manager, Brad Parscale, said in a statement. "And while Biden is the Democrat establishment's candidate, President Trump remains the disruptor candidate who has brought change to Washington. President Trump's supporters will run through a brick wall to vote for him. Nobody is running through a brick wall for Joe Biden."