His departure forces the progressive wing to face a choice: line up behind a candidate whose longstanding centrism they've denounced; or dig their heels in even as gears now shift towards the general election.
Amid the economic and political uncertainty of the coronavirus pandemic, the Democratic Party now must struggle to bridge the distance between their disparate factions.
And Monday, mere days after suspending his own campaign, Sanders endorsed Biden's candidacy, in a surprise campaign livestream appearance. Appearing in a split screen call for unity, it marked further heavyweight support behind the former vice president; Sanders' leadership holds especial clout amongst progressives.
"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 -- to make certain that we defeat somebody who I believe -- and I'm speaking just for myself now -- is the most dangerous president in the modern history of this country," Sanders said Monday.
"It's no great secret out there, Joe, that you and I have our differences. We're not going to paper them over -- that's real," Sanders said as he endorsed Biden, then adding, "we need you in the White House."
"I think that your endorsement means a great deal. It means a great deal to me," Biden said. "I think people are going to be surprised that we are apart on some issues, but we're awfully close on a whole bunch of others."
Biden has signaled he's working to woo those resistant progressive groups wary of the "establishment standard-bearer." Multiple sources told ABC News that for weeks already, the Biden team has been reaching out to a range of progressive groups and individuals -- long before Sanders dropped out -- and including some of those who had endorsed Sanders.
The two former rivals' staffs have recently been in talks to create task forces focused on key platform issues such as climate change, health care and the economy -- to find common ground between the two wings of the Democratic Party and the issues on which they've most diverged.
Wednesday, as Sanders left the race, Biden addressed both his former rival and his base in a lengthy letter, echoing Sanders' own campaign slogan.
"You will be heard by me. As you say: Not me, Us," Biden wrote. "And to your supporters I make the same commitment: I see you, I hear you, and I understand the urgency of what it is we have to get done in this country. I hope you will join us."
And as the party prepares to go toe to toe with Trump, moderates and erstwhile 2020 contenders -- 15 and counting -- have coalesced in support of Biden's candidacy. But some in the progressive wing have insisted that they will not go quietly into his big tent.
"This outbreak is changing the political landscape like we've never seen before ... it's an unprecedented time, but it's also a time for the Democratic party to decide what precedent they want to set for who, and what ideas, lead us next," Dana Fisher, a professor of sociology and the director of the program for Society and Environment at the University of Maryland, told ABC News.
Even as Sanders announced his support of Biden on Monday afternoon, some of his campaign staffers said their former boss does not dictate the terms of their support.
Sanders' former senior advisor Winnie Wong tweeted that Sanders could still be president. In short - it's not over yet.
“This is a controversial tweet but Bernie could still be president. We literally have no idea what the universe has in store for us. #COVIDー19.”
Sanders' former national press secretary openly said that she doesn’t support Biden.
"With the utmost respect for Bernie Sanders, who is an incredible human being & a genuine inspiration, I don't endorse Joe Biden," Briahna Joy Gray tweeted on the heels of Sanders' endorsement. "I supported Bernie Sanders because he backed ideas like #MedicareForAll, cancelling ALL student debt, & a wealth tax. Biden supports none of those.”
And Sanders' staff is not alone in this resistance: a sign that Sanders' support, while significant, does not steer the movement he built.
In the wake of Sanders' suspension and prior to his endorsement, youth activists groups also took to Twitter, to make it clear that just because there's one candidate left does not mean he's theirs, in unison tweeting their resistance to the party's urging to unite: "we are not endorsing Joe Biden."
Multiple progressive group leaders told ABC News that if he wants their support, there's still work to be done; the Biden team knows it, and has set about signaling they're on board.
Now, it's a matter of whether voters will be, too; netting further progressive support may require some strategic trailblazing from both sides in what was already an unprecedented hour of need.
Leading through a crisis
Sanders himself conceded his path to the nomination had come to a close in part because of the coronavirus' pandemic. Wednesday afternoon he told supporters via livestream, the need for a united effort to defeat the virus, compounded by the lack of leadership from the current White House incumbent, expedited his exit.
"As I see the crisis gripping the nation, exacerbated by a president unwilling or unable to provide any kind of credible leadership, and the work that needs to be done to protect people in this most desperate hour, I cannot in good conscience continue to mount a campaign that cannot win, and which would interfere with the important work required of all of us in this difficult hour," Sanders said.
"This moment that we're in is not political. This is a public health crisis," Symone Sanders, a senior adviser to Biden's presidential campaign who is also spearheading their progressive outreach, told ABC News. "And in moments like these, Americans have to come together."
But coming together means somebody has to move.
And if the former vice president is hoping to gain their support -- progressive groups say, that somebody better be him. And the move better be a shift left, both in terms of platform and personnel.
On the heels of Sanders' exit, eight progressive groups penned an open letter to Biden arguing that his campaign has "not earned the support and trust" of young voters, proposing a series of policies and actions he could take to win them over: more liberal stances on climate change; gun control; health care.
"We're highlighting areas for growth, opportunity, and some to really engage a highly ready for change group of young people across the country," March for Our Lives' Executive Director Alexis Confer told ABC News. She confirmed that the Biden campaign had been in touch with them in weeks prior regarding their platform of gun violence prevention -- and that they had been "very receptive."
Multiple progressive leaders expressed to ABC News that Biden must look to shore up his left flank; without it, his position to beat Trump is tenuous at best.
"Of course he's got a shot but it's probably a coin flip and I think everything he does now is going to matter," national director of the Sunrise Movement Evan Weber said. The Biden campaign had also approached his group in recent weeks, Weber confirmed, to "acknowledge the work still needing to be done."
"It's going to be a monumental task to mobilize and persuade some voters to turn out, especially when the situation in this country is so dire and confusing," Weber said.
An uphill climb perhaps -- though one the Biden camp has set upon for weeks.
A source close to the climate council of Washington Gov. Jay Inslee, a former 2020 presidential candidate who pushed the entire field toward more progressive approaches on climate change, told ABC News that several members of their teams have had two meetings, just within the last month: "productive conversations," addressing concerns of both specific policy and also, prioritization of climate in the former vice president's agenda, and in how he builds his staff infrastructure.
"We're giving a pretty clear roadmap on how to earn our vote," Yonah Lieberman, spokesperson for If Not Now, one of the progressive groups cosigning the letter, told ABC News.
The Biden campaign had not reached out to them, Lieberman said.
"The ball's in their court to put in the work necessary or act like it's locked up," he said. "And we know that was one of the mistakes of Hillary's campaign in 2016."
Youth movement and progressives' support hinges on moving Biden's needle toward a more left-leaning set of policy prescriptions.
Though Biden overwhelmingly won in Super Tuesday contests, Sanders won voters younger than 30 by a smashing 58-13% over Biden and those age 30 to 44 by 44-20%. "Very" liberal voters backed him by 47-18%, independents by 38-24%.
On one of his signature issues, Sanders won voters who support a government-run, single-payer health care system -- 56% of all those who voted -- by 43-22% over Biden.
And a source close to Massachusetts Sen. Elizabeth Warren's former presidential campaign told ABC News that when it came to her endorsement -- one of the last still up for grabs -- her support needed to be earned, but that she will support the party's nominee.
The strategy of no
Amid the Biden back-channeling with progressive groups, the organizations' public letter Wednesday was meant as a "show of solidarity."
Sources familiar with the progressive groups' strategy on this front say they gave the Biden team advance notice they were putting this letter out and that the move was about exerting the leverage.
That follows the route Sanders himself now appears to take: he has suspended his campaign, pledging to work with the presumptive nominee to push his progressive agenda, and endorsed Biden in that role.
However, he has also said he will keep his name in remaining races in order to collect delegates for the Democratic convention, "where we will be able to exert significant influence over the party platform."
His new support for Biden does not blot his name from the upcoming ballots.
"Now is the time to apply pressure," Heather Greven, a spokeswoman for NextGen America, a progressive advocacy group, told ABC News.
As a super PAC, her organization cannot under Federal Election Commission laws coordinate with candidates or their campaigns, thus they had not been in touch with the Biden team.
However, a joint public comment helps push the conversation and the candidate forward, Greven said.
"This is the time where you ask for everything you can get, and push as hard as you can," Greven continued. "It's going to take investment from their campaign, and not just a verbal investment -- that's a great sign of life, and you know, his recent policy movements. But how do you eat an elephant? One bite at a time."
Leading up to and directly following Sanders' official exit, Biden announced his support of several progressive platform points, including Warren's plan to make it easier to file for bankruptcy, as well as end provisions making it difficult for student loans to be discharged while filing for bankruptcy.
Biden's adoption of Warren's plan marks a massive change for the former Delaware senator, as it would undo much of the Bankruptcy Abuse Prevention and Consumer Protection Act of 2005 -- a measure the two have locked horns over for the better part of two decades.
Hours after Sanders suspended his campaign, Biden was asked at a virtual fundraiser how he planned to engage young voters who cared about climate change. Biden described his policy and added he had asked his staff to work on expanding several of his policy proposals on climate change so that "good ideas, wherever I can find them on every issue, are brought into the campaign."
The day after, Biden announced further support for student debt forgiveness and lowering the Medicare age requirement.
"Entering the next phase of the election, we are very focused on intentional and carefully curated engagement," Symone Sanders told ABC News. "We want to be sincere with our progressive engagement. But we also know that it's nuanced - it's not a one size fits all policy.
"We're approaching these conversations with progressive individuals and groups," she added. "Not to say, where can Vice President Biden move on your issue? But more so, where are we both at? And, where do we agree?"
Will such olive branches reach far enough?
For some progressives though, the olive branch does not reach far enough.
Before Sanders dropped out, the hashtags #NeverBiden and #WriteinBernie trended on Twitter, pushed by some members of the progressive left. And after his departure, #WhyIwon'tvoteforBiden was trending, echoed by Sanders supporters and his own staff.
ABC News spoke with a number of voters using this rhetoric, who rattled off a myriad reasons for why they don't want the former vice president back in the White House.
To them, he represents establishment politics when they're calling for a change of the guard and a party that remains stubbornly centrist and blind to the real needs of future generations.
"It's not just about him -- it's what the party doesn't understand -- what's becoming even more imperative: we're looking at millions of people now that may be left with nothing to lose in the next two weeks," Alyson Metzger, a writer and progressive activist said before Sanders' exit from the race and reiterated after his departure. "This man (Biden) has been on the opposite side of every progressive issue; what's really critical is to break this bubble of the ruling class and the establishment."
Sanders' support does not sway that sentiment now.
"Nobody listened, including Bernie apparently," Metzger said after Sanders' endorsed. "Endorsing this early just makes (Sanders' supporters) less likely to support Biden."
Others still stinging from Bernie's bitter loss in 2016, resent the presumption that they'll just fall in line as Biden takes the party's reins, especially since they don't think he'd steer Democrats towards a win in 2020. The current moment of crisis, they say, makes Biden an even worse option.
"I think people are getting more and more anxious in this outbreak, and we need a leader," Scott Menor of Arizona, tech entrepreneur with a doctorate in physics, told ABC. "I know people are really desperate for a steady hand, or a return to normalcy, but what I see doesn't give me a great deal of confidence and I see a lot of opportunities for someone to spin what he says against him."
"It's about risk and probability: you've got a lot of stuff in (Biden's) history that seems extremely problematic and dangerous," Menor said, pointing to longstanding concerns by many of how Republicans might raise questions about Hunter Biden.
The former vice president's son has faced criticism for his former ties to a Ukrainian gas company, Burisma. The Bidens have repeatedly denied wrongdoing and have not been accused of doing anything illegal; however his role on the board presented the appearance of a conflict of interest, ethics experts have told ABC News.
"That's an arrow that's removed from our quiver and even Democrats will start saying, 'Eh, he's better, but not that much better than Trump.' And we lose the ability to win," Menor continued. "People picked Biden because they think he's the safe bet -- the problem is, I don't think Biden is really that safe."
Those concerns have not dissolved as the Democratic field has winnowed down to one.
"You may not know it til November," Menor said the day Sanders dropped out, "but today is very likely the day Trump got reelected."
"Trump has something like 90% approval rating amongst Republicans," podcast host Benjamin Dixon told ABC News this week. He has not explicitly said he's in the ‘Never Biden' camp, but strongly supported Sanders. "Biden has an enthusiasm gap. Even Bernie Sanders was going to have the fight of his life defeating trump. Good luck, Joe."
"I'm glad I ordered the good Scotch," Metzger said when Sanders endorsed.
Brewing resentment from feeling taken for granted now foments some progressive reticence to go with the flow, even as the Democratic Party forges ahead. For some, a pledge never to vote for Biden now is an effort to gain that same leverage for a voting bloc that has felt marginalized.
Metzger told ABC News there is "no circumstance" under which she would vote for Biden, even with his latest overtures, which she finds difficult to trust.
This isn't about being intractable, Metzger said, it's about showing their swath of votes should not be taken for granted -- and pushing back against the feeling they're being bullied with the blackmail of Trump as an alternative.
"We're trying to raise the alarm -- to tell the party -- wake up! If you don't move to us, we're not going to vote," Metzger continued. "In 2016 most people sucked it up and pulled that lever for Hillary. Not again. Saying you will "vote blue no matter who" is a guaranteed way for the party to ignore you."
The threat may be strategic saber-rattling, Democratic strategists and progressive voters admit: in objecting vehemently to Biden, they may look to wield political peer pressure on this side of the primary -- to move his needle on the issues now.
"So my thinking is kind of game theory - part of the calculation I think is people like me will just come along for the ride," Menor said. "They can just count on me to vote for Biden in the general, so you don't really have to factor in my opinion. And I want them to know that no, you can't! You can't count on me. And in fact, you can count on me not to support him," Menor said.
That's a stance that may shift as November draws nearer -- and as the COVID-19 pandemic grows more grim, progressive leaders and political experts say.
Dixon told ABC News, health care is a deal maker -- or breaker -- for him: Biden would need to support "Medicare for All." "If he supports that I'll vote for him," Dixon said, adding, he understands Sanders' endorsement, but for him, it's about the issues.
"The last thing (Sanders) wants on his conscience is having helped Trump in any fashion," Dixon said. "But that’s not enough for people fighting for specific policies that Biden is fighting against. We’re the engine of economic prosperity so we’re demanding what we need. Bernie did what he could. We love him for what he did, but the movement is clearly saying they’re moving on without him."
"Resisting alone, you know, is like bowling alone," Fisher said, adding she feels it may be a futile effort. "And the party may look to unite. Many right now may not be swayed yet, and some of them are digging in, and I get that. But -- getting to the election from here, given what we know about the virus -- it may be disastrous. So, I think people could change their opinion down the road."
The issue may be less one of a protest vote and more a lack of passion that won't push voters to engage their cohort to turn out on Election Day.
The "Get Out the Vote" effort, so core to the grassroots Democratic effort, has already been hampered by the coronavirus' spread with campaigning, rallies and town halls all unfolding in the virtual space.
However, such online efforts could be further dampened by a lack of enthusiasm for some over having Biden at the top of the ticket.
"How far are progressives willing to go to elect a moderate candidate? It's not that progressives won't vote for Joe -- they'll certainly vote for anyone but Trump," said Fisher who has researched and collected data on the youth climate movement at length. "But that doesn't mean that they'll knock doors and kill themselves to get out the vote. The problem is, this candidate may not instill the same enthusiasm for them."
One Democratic strategist who has had close ties to the Warren campaign told ABC News, the threat to stay home is likely "just one of the stages of grief."
"There is a whole group of people in this country who really feel like the political system is pointless, and doesn't work or speak for them," Sunrise Movement's Evan Weber said. "Folks are angry and confused, even thinking about potentially voting third party or giving up on electoral politics altogether -- like what's the point in voting if the establishment is just going to completely ignore the things we're crying out for, and not cater to us in any way, or blatantly try to quash what we're fighting for?"
"The pain that they're speaking from is real. And it's worth listening to, even if it's hard for people to understand - or they don't necessarily like the way that it's expressed."
But even that type of sentiment could undermine the Democratic party's efforts heading into the general election, political experts and strategists said.
For his part, Biden's campaign has emphasized the importance of not counting anyone out.
"We're all on the same team. It is important for us to take a step back and say, this is a campaign for everyone," Symone Sanders told ABC News. "There is a place here for you."
ABC News' Political Director Rick Klein, Halimah Abdullah, Johnny Verhovek, Molly Nagle, Averi Harper and Adam Kelsey contributed to this report.