Bernie Sanders has endorsed his former rival Joe Biden for the US presidency in a joint online appearance.

The backing is a crucial development for Mr Biden, who must bridge the Democratic Party’s ideological divide to unify voters against Republican president Donald Trump.

Mr Biden and Mr Sanders, a leading progressive, clashed throughout the primary over policy issues such as the “Medicare for All” universal health care plan.

“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,” Mr Sanders said.

The endorsement stands in contrast to the extended 2016 fight between Mr Sanders and Hillary Clinton, who became the nominee that year.

He only endorsed her at the end of a drawn-out nominating fight and amid a bitter fight over the Democratic platform that extended to the summer convention.

Appearing in a split screen with Mr Biden, Mr Sanders said there is “no great secret out there that you and I have our differences”.

But he cited ongoing work between the two camps on several policy matters as a reason for the endorsement.

He added the biggest priority was defeating Mr Trump in November.

“We’ve got to make Trump a one-term president,” Mr Sanders said.

“I will do all that I can to make that happen.”

The backing came less than a week after Mr Sanders ended his presidential campaign.

The coronavirus prevented Mr Biden and Mr Sanders from appearing together in person on Monday.

But they made clear they would continue working together, announcing the formation of six “task forces” made up of representatives from both campaigns to work on policy agreements addressing health care, the economy, education, criminal justice, climate change and immigration.

Mr Biden, 77, has already made some overtures to progressives by embracing aspects of Mr Sanders’ and Senator Elizabeth Warren’s policies.

The day after Sanders exited the race, Mr Biden came out in support of lowering the Medicare eligibility age from 65 to 60 while pledging to cancel student debt for many low- and middle-income borrowers. He has also previously embraced Ms Warren’s bankruptcy reform plan.

Mr Sanders, 78, is sure to remain a force throughout the campaign.

When he ended his candidacy, he said he would keep his name on the ballot in states that have not yet voted in order to collect more delegates that could be used to influence the party’s platform.

He did not say on Monday whether he would continue to fight for those delegates.

Election 2020 Debate
Bernie Sanders has endorsed his former rival Joe Biden (Evan Vucci/AP)

Still, Mr Sanders and Mr Biden emphasised their mutual respect for each other on Monday.

Mr Sanders referred to the former vice president as “Joe”.

Mr Biden answered him repeatedly as “pal”.

The two men asked the other to give regards to their wives, Jill Biden and Jane Sanders.

Mr Biden told Mr Sanders: “I really need you, not just to win the campaign but to govern.”

Mr Trump’s campaign, meanwhile, seized on Mr Sanders’ endorsement to underscore Mr Biden’s embrace of some of his plans, with campaign manager Brad Parscale saying in a statement that “though Bernie Sanders won’t be on the ballot in November, his issues will be.”

Mr Biden and Mr Sanders also emphasised the need to address the challenges confronting young people during the pandemic, with Mr Sanders describing “a generation of young people who are experiencing crisis after crisis”.