Sanders first made the announcement in a call with his staff, his campaign said.
"I wish I could give you better news, but I think you know the truth, and that is that we are now some 300 delegates behind Vice President Biden, and the path toward victory is virtually impossible," Sanders said in a livestream after the call. "So while we are winning the ideological battle and while we are winning the support of so many young people and working people throughout the country, I have concluded that this battle for the Democratic nomination will not be successful. And so today I am announcing the suspension of my campaign."
Sanders' exit caps a stunning reversal of fortune following a strong performance in the first three states that voted in February. The nomination appeared his for the taking until, on the last day of February, Biden surged to a blowout victory in South Carolina that set off a consolidation of moderate voters around the former vice president. The contest ends now as the country continues to grapple with the coronavirus pandemic, which halted in-person campaigning for both Sanders and Biden and has led many states to delay their primary elections.
Sanders said he did not make the decision lightly, describing it as a "very difficult and painful decision."
"Over the past few weeks Jane and I, in consultation with top staff, and many of our prominent supporters, have made an honest assessment of the prospects for victory. If I believed we had a feasible path to the nomination, I would certainly continue the campaign. But it's just not there," he said.
Sanders' departure from the race is a sharp blow to progressives, who rose up during and after the 2016 campaign and commanded the Democratic Party's Trump era debates over issues like health care, climate change and the effects of growing economic inequality.
"Few would deny that over the course of the past five years, our movement has won the ideological struggle," Sanders said on Wednesday. "It was not long ago that people considered these ideas radical and fringe. Today, they are mainstream ideas. Many of them are already being implemented in cities and states across the country."
But even as his policies grew more popular over the years and into the primary season, the Vermont senator struggled to broaden his own support and galvanize a winning coalition. Now, as he did after leaving the 2016 primary, Sanders will seek to influence the presumptive nominee through the means he knows best -- from the outside-in.
Biden has already made gestures toward Sanders' populist base, which formed a movement over the past five years that could be critical to defeating Trump in the fall. Whether the former vice president will take the necessary steps to win over the holdouts, and the extent to which Sanders goes to make the case, will be a running subplot until Election Day. Sanders' decision to remain on the primary ballot in order to amass "as many delegates as possible" as part of an effort, he said, "to exert significant influence over the party platform and other functions" at this summer's convention underscores the depth of the divisions both he and Biden still have to bridge.
Sanders and Biden spoke Wednesday morning, with the Vermont senator telling the former vice president about his decision to end his presidential campaign, according to people familiar with the call.
In a statement after Sanders' announcement, Biden called the senator a "powerful voice for a fairer and more just America" and said his campaign's impact on the election is far from over. He also made an explicit call for Sanders' supporters to join him.
"And to (Sanders') 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. You are more than welcome. You're needed," Biden said."
Sanders also acknowledged on Wednesday that some of his supporters would be disappointed by his exit.
"I know that there may be some in our movement who disagree with this decision, who would like us to fight on until the last ballot cast at the Democratic convention. I understand that position," he said. "But 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."
The pivot from four- and five-event days came abruptly last month as Americans attempted to combat the coronavirus by staying, sometimes on state and local leaders' order, inside their homes.
The Sanders campaign held its final live public event on March 9, transitioning from packed, raucous rallies to an entirely digital operation. He communicated almost exclusively through virtual town halls and livestreams focused on the coronavirus crisis -- and how his progressive agenda, headlined by "Medicare for All," might have prevented it or helped cushion the blow.
It was a point he returned to on Wednesday morning.
"This current, horrific crisis we are now in has exposed for all to see how absurd our current, employer-based health insurance system is," Sanders said. "The current economic downturn we are experiencing has not only led to a massive loss of jobs but has also resulted in millions of Americans losing their health insurance."
Even in the end, the message remained the same. For Sanders' supporters, though, the context was jarring.
Less than two months ago, the Vermont senator appeared poised to run away with the nomination after a strong performance in Iowa and victories in New Hampshire and Nevada, the latter by more than 25 percentage points, on the strength of his popularity with Latino voters, which had been courted relentlessly by his campaign.
But Sanders' momentum was dashed in South Carolina. Biden routed the field and then cleared it. The anti-Sanders vote rallied around him and, even with Sanders' win in California, put Biden in the driver's seat on Super Tuesday.
The wind at his back, the former vice president duplicated the feat a week later, delivering the hammer blow in Michigan, a state Sanders won in 2016 and viewed as crucial to his prospects in 2020. A day earlier, public safety measures in response to the coronavirus effectively ended the campaign roadshow.
Sanders would return to Vermont, where he has spent most of his time since, while Biden set up headquarters at home in Delaware. The Sanders fundraising machine, the most successful grassroots donor effort in American political history, was over the last month re-purposed into a feeder for public health groups.
Fall and rise, and fall
Sanders entered the race in February 2019 as an early frontrunner. In an email announcing his second run for the presidency, he asked voters to "join me today as part of an unprecedented and historic grassroots campaign that will begin with at least a million people from across the country."
He would quickly hit and surpass that goal and raise, throughout the campaign, more money from small dollar donors than any candidate in American political history.
His status atop the early polls, before Biden got in the contest and claimed the lead, underscored the power Sanders had built up since beginning his first presidential campaign as a little-known lawmaker from Vermont who freely called himself a democratic socialist.
By the end of the 2016 race, Sanders emerged as one of the most influential figures in Democratic politics. His policy agenda -- a suite of progressive proposals including Medicare for All, tuition free public college and the Green New Deal -- set the terms of debate among the 2020 candidates.
Despite entering with a head start on the field, Sanders' second campaign encountered some early headwinds as Democratic voters sampled a diverse array of candidates, many of them offering pieces of the progressive vision that Sanders popularized. By the late summer, Sanders appeared to be falling behind fellow progressive Elizabeth Warren, his Senate colleague from Massachusetts.
A campaign on its heels would be knocked to the pavement in early October, when Sanders had a heart attack while campaigning in Nevada.
But what many supporters and staff feared would be the end turned out to be a springboard. Sanders recovered quickly and, before leaving Las Vegas, received a call from Rep. Alexandria Ocasio-Cortez.
The young, progressive star, who had been intensely courted by Warren, told Sanders she was endorsing him. Her early events for Sanders in Iowa were electric, but as the primary heated up, her appearances become fewer and further in between -- an early sign of the tactical tensions that would face the left in the coming months.
Still, for a campaign that had scuffled through the summer and into the fall, it felt -- at least for that moment in mid-October moment -- as if a switch had been flipped.
Sanders returned to the trail only a couple weeks after falling ill and, after turning in a sterling debate performance in Ohio on October 15, began to climb in the polls.
Days later, in a packed riverside park in Queens, New York, after being introduced by Ocasio-Cortez and taking the stage to AC/DC's "Back in Black," Sanders addressed one of his largest and most diverse audiences of the campaign.
"Take a look around you and find someone you don't know. Maybe somebody that doesn't look kinda like you, maybe somebody who might be of a different religion than you, maybe they come from a different country," he said.
Aides who had driven and flown in from around the country looked on, rapt like his supporters, some of them watching from across the streets after the park hit its capacity.
"My question now to you is are you willing to fight for that person, who you don't even know," Sanders said, "as much as you're willing to fight for yourself?"
The question -- and the call -- would be a centerpiece of his campaign in the weeks and months that followed.
Ocasio-Cortez, who looked on as he said those words, tweeted in praise of Sanders after the decision was made public.
"Thank you Bernie - for doing your best to fight for all of us, from the beginning, for your entire life," she wrote. "Thank you for fighting hard, lonely fights in true devotion to a people's movement in the United States. Thank you for your leadership, mentorship, and example. We love you."
A quick unraveling
As Biden, in the aftermath of South Carolina, lined up support and endorsements from moderates across the board, Sanders was unable to do the same with the progressive wing of the party. Warren, buoyed by the late-arriving support of a super PAC, stayed in the race through Super Tuesday.
She dropped out the following Thursday, but despite being so closely aligned with Sanders on policy, chose to stay on the sidelines instead of putting her support behind his struggling campaign. Though some Sanders' supporters lashed out at Warren for not immediately backing him, the writing had been on the wall.
The pair had been close, as political figures go, though they took fundamentally different views of how to win and exert power. But the controversy over whether Sanders told Warren in late 2018 that he did not believe a woman could win the presidency -- which she confirmed and he denied -- cut off meaningful communications between the campaigns.
In the end, and as many on both sides acknowledged privately at the time, it was already too late.
Warren on Wednesday tweeted in praise of Sanders and the impact of his two presidential campaigns.
"Your fight for progressive ideas moved the conversation and charted a path for candidates and activists that will change the course of our country and party," she wrote.
After losing five of the six contests on March 10, including Michigan, and all three primaries on March 17, Sanders campaign manager Faiz Shakir signaled the beginning of the end. Sanders, he said in a statement, "is going to be having conversations with supporters to assess his campaign," but that "in the immediate term, however, he is focused on the government response to the coronavirus outbreak."
Sanders batted away questions about the future of his campaign through the latter half of March, as his campaign -- with the exception of a handful of combative surrogates and staff who continued to batter Biden -- largely retrained its focus on the coronavirus.
Asked about his plans during a recent visit to Capitol Hill, Sanders bristled, telling CNN, "I'm dealing with a f**king global crisis." By then, his livestream conversations and other campaign-adjacent online events rarely made mention of the primary or Biden.
As Sanders started to make more television appearances after leaving Washington, he became increasingly frank about his chances of winning the nomination.
"There is a path, it is admittedly a narrow path," Sanders told "Late Night" host Seth Meyers last week.
But those acknowledgments were mixed in with public arguments for staying in the race -- and other remarks, more difficult to discern, that offered some insight into Sanders' own indecision.
"I mean, right now, under normal times, I wouldn't be talking to you from my home" Sanders told CNN's Anderson Cooper in an interview on Friday. "So yes, the calculus has absolutely changed. And you know, we're talking to a lot of people, and trying to figure out the best way forward."
Toward the end, the questions often appeared to shape the answers. After being pressed by Whoopi Goldberg on "The View" last week over his reasons for remaining in the race, Sanders sounded like he was settling in for a longer haul.
"Last I heard, people in a democracy have a right to vote," he said, "and they have a right to vote for the agenda that they think can work for America."
But in his livestream events, conducted with top aides, advisers and friends, Sanders was more expansive.
The pandemic, he argued, had pulled the tide out and revealed in the starkest ways possible the ugliest inequities in American life.
"I think it is not inappropriate to be trying to ask ourselves, how did we get to where we are today, and maybe where we want to go when all this is behind us," Sanders said on April 4. "And I think some of the questions that we have to ask ourselves, and you have heard me say this a million times, is how does it happen that we are the only major country on earth not to guarantee health care to all people as a human right?"
He spoke with former President Barack Obama in the last few weeks as he determined the future of his campaign, a source familiar with the conversation tells CNN.
Obama, throughout the 2020 Democratic primary, had regular conversations with candidates, including before, during and after their respective bids.
"His private counsel consistently emphasized staying focused on the ultimate goal: Winning the White House in November," the source familiar with Obama's calls tells CNN. "He was impressed by the caliber of our candidates and the strong campaigns they waged -- but always urged them to keep in mind that we must be well-positioned to unify as a Party once we have a nominee."
Five years after Sanders launched a message campaign with the hope of rejuvenating progressive grassroots and keep Hillary Clinton accountable to the Democratic Party's left flank, and after that bid touched off a movement that has spawned a new generation of leftist leaders, Sanders by the end of his 2020 race had, in many ways, returned to his own beginnings.
Whether Sanders' decision to the leave the contest now, rather than carrying on as he did in 2016 through the end of the primary calendar, will earn him some goodwill with the party establishment he fought so long and hard to upend, is an open question. An earlier departure won't blot out the ideological divisions that have roiled the party since 2016.
But the more immediate question facing Sanders, following his departure, and his supporters is whether and to what extent they will lend their support -- and organizing energy -- to Biden's campaign.
Sanders has been insistent that he would support the eventual nominee, no matter who it was. But his political base -- especially the young, who voted for him by overwhelming margins, and disaffected -- will be more difficult to bring along, no matter how many miles Sanders covers on Biden's behalf.
This story has been updated.
CNN's Kate Sullivan, Jeff Zeleny, Sarah Mucha and Dan Merica contributed to this report.