Bernie Sanders endorsed his former rival Joe Biden for president on Monday, during a surprise appearance on a live stream related to the US response to the coronavirus pandemic.

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Sanders’ support of the former vice-president comes at a critical moment in the 2020 campaign, as Biden attempts to unify the party after a fiercely contested primary. By throwing his weight behind Biden, Sanders, is sending a clear signal to his legions of loyal followers that they should align behind the party’s nominee to defeat Donald Trump.

“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,” the senator from Vermont said, appearing via live stream on a split screen next to Biden, who was hosting the broadcast.

“We need you in the White House,” Sanders said. “We’ve got to make Trump a one-term president. I will do all that I can to make that happen.”

Sanders dropped out of the Democratic primary race last week after falling far behind Biden in the delegate count. In an effort to continue influencing the Democratic platform, he will remain on the ballot in states with remaining primaries to gain delegates.

Biden, a former Delaware senator and vice-president, now the presumptive Democratic nominee for the White House, thanked Sanders for his support.

“You’ve been the most powerful voice for a fair and more just America,” Biden told Sanders. “You don’t get enough credit, Bernie, for being the voice that forces us to take a hard look in the mirror.”

“If I am the nominee – which it looks like now you just made me – I really need you, not just to win the campaign but to govern,” Biden told Sanders.

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Biden still faces deep skepticism from young progressive voters, many of whom supported Sanders by lopsided margins. On Monday, Biden and Sanders announced that their staff had set up six taskforces to help find common ground on policy differences. Those groups will cover the economy, education, criminal justice, immigration, climate change and healthcare.

“It’s no great secret out there, Joe, that you and I have our differences. And we’re not going to paper them over, that’s real,” Sanders said. But he expressed an optimism that the working groups would be able to forge policy agreements and concessions.

Determined not to repeat the mistakes of 2016, when the party struggled to unify, Biden has already started to ramp up his outreach to the party’s left. In recent weeks, he has engaged top progressive organizations and movement leaders representing a range of Democratic causes from climate change to racial justice.

He has also made overt gestures, adopting a plan by his former rival, Elizabeth Warren, to overhaul the consumer bankruptcy system, embracing elements of Sanders’ tuition-free college proposal and unveiling a plan to lower the age of eligibility for the Medicare government health insurance program for seniors from 65 to 60.

Though there are indications Biden will continue to move left on issues like climate change, he is unlikely to adopt Medicare for All, Sanders’ signature policy. Asked recently if the coronavirus outbreak changed his position on universal healthcare, Biden said it hadn’t and argued expanding the Affordable Care Act was still the most effective and efficient way to insure more Americans.

In a sign of the work that lies ahead, the progressive New York congresswoman Alexandria Ocasio-Cortez, one of Sanders’ most high-profile surrogates, called Biden’s Medicare proposal “almost insulting” and said he would have to go much further if he expects a full-throated endorsement.

“The whole process of coming together should be uncomfortable for everyone involved – that’s how you know it’s working,” she said in an interview with the New York Times. “And if Biden is only doing things he’s comfortable with, then it’s not enough.”

There are also limits to Sanders’ ability to deliver the support of his devoted followers, particularly those attracted to his outsider status as an independent railing against the political establishment. Though Sanders has been careful not to attack Biden in personal terms, some of his most visible backers continue to assail the former vice-president, raising questions about his mental acuity and overall fitness – a line of attack that Trump has also used against his likely opponent.

Despite a hard-fought primary, the former competitors and one-time Senate colleagues displayed sincere affection for one another – a warm relationship Sanders never shared with his 2016 rival, Hillary Clinton.

“I appreciate your friendship, and I promise you I will not let you down,” Biden told Sanders.

In reply, Sanders said: “I know you are the kind of guy who is going to be inclusive. You want to bring people in – even people who disagree with you.”

After exchanging questions on raising the minimum wage, and the US economy, they joked that they should end the live-cast by playing a game of chess.

“We’ll bore everybody for a few hours,” Sanders said.