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.”

The Vermont senator 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 live stream, adding that he would work to elect Biden.

“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.”

Biden said that Sanders’ endorsement “means a great deal to me.”

“We’re awfully close on a number of issues,” Biden said, adding that, “When I’m the Democratic nominee, which looks like now you just made me. I’m going to need you, not just to run the campaign, but to govern.”

“I promise I won’t let you down,” Biden added moments later.

Sanders’ endorsement of Biden came five days after he ended his own presidential bid.

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

Sanders 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 a number of issues.

Following that exchange, the pair engaged in a question-and-answer session, with Sanders asking questions about policy and Biden providing responses.

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