Bernie Sanders endorsed his former rival, Joe Biden, for president on Monday in a joint online appearance.

"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.

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The backing is a crucial development for Biden, who must bridge the Democratic Party's ideological divide to unify voters against President Donald Trump in November. Biden and Sanders, a leading progressive, clashed throughout the primary over policy issues such as the Medicare for All universal healthcare plan.

The endorsement stands in contrast to the extended 2016 fight between Sanders and Hillary Clinton, who became the nominee that year. Sanders endorsed Clinton, but only after the end of a drawn-out nominating fight and amid a bitter fight over the Democratic platform that extended to the convention.

Sanders did not immediately address Monday whether he would continue to fight for delegates at state conventions around the country or whether he would simply use his newfound alliance with Biden to influence the nominee and the policy slate that he will present voters.

I'm speaking now about our nation's economic response to COVID -- and I'll have a special guest with me. Tune in: https://t.co/VTJPX6da0v — Joe Biden (@JoeBiden) April 13, 2020

Appearing in a splitscreen with Biden, Sanders said there's "no great secret out there that you and I have our differences".

But he cited continuing work between the two camps on several policy matters as a reason for the endorsement. And he said the biggest priority was defeating Trump.

"We've got to make Trump a one-term president," Sanders said. "I will do all that I can to make that happen."