US Sen. Bernie Sanders formally threw his weight behind presumptive Democratic candidate Joe Biden Monday, just days after exiting the White House race. 

Sanders said that Democrats must unite behind the former vice president if US President Donald Trump is to be defeated in November's nationwide election, imploring his supporters to back Biden.

"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, the progressive standard-bearer, said during a livestream hosted by Biden's campaign.

"Joe, I know that there is an enormous responsibility on your shoulders right now, and it's imperative that all of us work together to do what has to be done," he added.

Sanders ended his campaign last Wednesday following a string of disappointing performances in successive primary races.

But it was Sanders whose campaign galvanized droves of new voters who sought an alternative to the Democratic establishment, and he remains popular among young voters, Latinos and within left-leaning circles, even as his campaign was dealt significant setbacks at the polls.

Whether or not those voters will lend their support to Biden remains to be seen. But Sanders is hoping they will.

Biden thanked Sanders for his endorsement, calling it a "big deal."

"Your endorsement means a great deal to me," he said. "If I am the nominee, which it looks like just now you just made me, I am going to need you, not just to win the campaign, but to govern."

Biden's candidacy will not be formalized until this summer's Democratic convention, but given the coronavirus pandemic he proposed the possibility of holding the gathering virtually.