Sen. Bernie Sanders officially endorsed Joe Biden for president on Monday, joining the former Vice President as a surprise guest on a livestream video hosted online.

"We must come together to defeat the most dangerous president in modern history," tweeted Sanders announcing his appearance with Biden and referencing President Donald Trump.

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"Today," Sanders said to Biden during the broadacst, "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."

Watch it live: