Barack Obama has endorsed his former vice president Joe Biden, for the Democratic nomination to face Donald Trump in the presidential election.

In a video released on Tuesday, the former president invoked the coronavirus pandemic and told Americans that “now is the time for all of us to help where we can and be there for each other, as neighbours, as coworkers, as fellow citizens” but that the “spirit of looking out for one another” must also be reflected in the White House.

“That’s why I’m so proud to endorse Joe Biden,” he said. “He has all the qualities we need in a president right now. ... He has the character and experience to guide us through one of our darkest times and heal us.”

Until now, the former president has avoided publicly stepping into the race, but his endorsement follows reports that he had several conversations with Mr Biden’s chief rival, Bernie Sanders, to consider withdrawing, while Democratic insiders had attempted to lure Mr Obama to the fold as the Vermont senator saw a streak of primary election wins.

In his endorsement, Mr Obama hailed Mr Sanders and suggested that his progressive platform will be critical to Mr Biden’s success.

On Tuesday, Mr Obama called Mr Sanders an “American original” who has “devoted his life to giving voice to working people’s hopes, dreams and frustrations”.

Though he admitted they have not agreed, the two men have “always shared a conviction that we have to make America a more fair, just and equitable society”.

His supporters, particularly young people, “will be critical in moving America in the direction of progress and hope”.

“The Democratic party will have to be bold,” he said. “If I were running today, I wouldn’t run the same race or same platform as I did in 2008 ... We have to look to the future. Bernie understands that, and Joe understands that.”

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Mr Obama called for “vast infrastructural change”, the need for which the pandemic has laid bare, and stressed that American workers “have always been essential” while also being “underpaid” and “financially stressed with too little support” as young people prepared to graduate “into unprecedented unemployment”.

Healthcare must be “affordable to everyone” and lawmakers must “finish the job so health care is not just a right but a reality for everybody”, he said.

But he stressed that Democrats face a powerful Republican party led by the president, armed with a significant war chest and propaganda network.

He said: “Republicans in the White House and running the US Senate are not interested in progress. They’re interested in power.”

The right is “willing to kick millions from their health insurance and eliminate pre-existing conditions for millions more ... even in the middle of this public health crisis” while also proposing tax cuts for wealthy Americans and giving “polluters unlimited power to poison our air and water” while denying the “science of climate change just as they denied the science of pandemics”, he said.

“Repeatedly they’ve disregarded American principles, of rule of law and voting rights, and transparency — basic norms that previous administrations observed regardless of party”, he said. “Our country’s future hangs on this election.”

Following his endorsement announcement, Mr Biden told his former boss on social media: "This endorsement means the world to Jill and me. We're going to build on the progress we made together, and there's no one I'd rather have standing by my side."

The pandemic has carved out an unprecedented election schedule with an unpredictable chain of events, with non-traditional campaigning like virtual endorsements likely becoming the norm through the fall.

Most of the remaining primary elections have been postponed through the summer, and the Democrats' nominating convention — where delegates will formally choose their candidate — won't be held until August.