In this video still image from the Bernie Sanders Presidential Campaign, Sanders announces the suspension of his presidential campaign on Wednesday, from Burlington, Vermont. Photo: AFP

Former White House candidate Bernie Sanders on Monday endorsed onetime rival Joe Biden for president, saying it was time to unite in the effort to defeat Donald Trump in November.

The two veteran politicians, who each spent the past year battling for the Democratic nomination, appeared together by split-screen on Biden's livecast.

"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.

The swift backing marked a notable change from 2016, when Sanders lost the presidential nomination race to Hillary Clinton but delayed endorsing her candidacy. That wait was seen as deeply damaging in that it exposed a divide within the Democratic Party.

Sanders struck a unifying tone on Monday, telling viewers "it's imperative that all of us work together."

The 78-year-old independent ­leftist senator from Vermont exited the 2020 race last week after losing almost all of the primary contests following a strong start, leaving Biden as the party's ­presumptive nominee.

The deadly coronavirus pandemic has forced a suspension of all in-person political campaigning, as most of the nation remains under stay-at-home orders - including Biden and Sanders, who appeared online from their homes.

Sanders on Monday signaled it was crucial to unite around Biden's campaign to defeat Trump, whom Sanders branded "the most dangerous president in the modern history of this country."

Biden, who at 77 is mounting his third presidential run, said he was "deeply grateful" for the endorsement - one of many that have come from former campaign rivals including Senator Kamala Harris, Senator Amy Klobuchar and former congressman Beto O'Rourke.

Biden announced the two men were establishing several policy working groups to address issues like climate change, healthcare, economic justice and college affordability.