Sen. Bernie Sanders Bernie SandersMcConnell accuses Democrats of sowing division by 'downplaying progress' on election security The Hill's Campaign Report: Arizona shifts towards Biden | Biden prepares for drive-in town hall | New Biden ad targets Latino voters Why Democrats must confront extreme left wing incitement to violence MORE’s (I-Vt.) supporters are deeply disappointed over the way his presidential run was snuffed out, first by former Vice President Joe Biden Joe BidenCast of 'Parks and Rec' reunite for virtual town hall to address Wisconsin voters Biden says Trump should step down over coronavirus response Biden tells CNN town hall that he has benefited from white privilege MORE and then by the coronavirus pandemic.

Only six weeks ago, Sanders and his allies could smell victory, not just for the candidate but also for a progressive movement eager to establish itself in the mainstream.

What started out as a sparsely attended official election launch in 2015 from the lawn outside the Capitol in Washington, D.C., grew into a political force that reshaped the Democratic Party, briefly turning Sanders into the front-runner for the party’s 2020 presidential nomination.

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But instead of ending with Sanders’s coronation in Milwaukee later this summer, the Vermont senator ended his campaign on Wednesday in a video address from his home after a string of dispiriting primary defeats and a global pandemic that ground his campaign to a halt.

“It’s a huge disappointment,” said Bill Press, one of a small cadre of advisers Sanders consulted before launching his 2016 presidential bid. “After doing better than anyone could have possibly imagined in 2016, he came into 2020 with a head full of steam. It was going so well, moving so fast, and then there was a perfect storm that gathered against him.” Press is a columnist for The Hill.

Sanders won the New Hampshire primary in late February and then steamrolled the field in the Nevada caucuses on the strength of what looked to be a burgeoning coalition of young people, working-class voters and Latinos.

From there, the Vermont senator opened up double-digit leads in national polls, cut into Biden’s lead among black voters and appeared poised to build a potentially insurmountable lead in delegates if he could win big in California on Super Tuesday.

Heading into March, Sanders was the odds-on favorite to win the nomination. The news media was in a frenzy over the potential for a historic battle of outsiders between Sanders and President Trump Donald John TrumpHR McMaster says president's policy to withdraw troops from Afghanistan is 'unwise' Cast of 'Parks and Rec' reunite for virtual town hall to address Wisconsin voters Biden says Trump should step down over coronavirus response MORE in November.

But mainstream Democrats and Sanders’s rivals sounded the alarm, warning that the party would face certain defeat by nominating a democratic socialist.

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The tide turned against Sanders with astonishing force, resulting in Biden pulling off one of the most stunning political comebacks in presidential primary history.

Biden won South Carolina in landslide fashion and then rolled through Super Tuesday on the strength of his support from black and moderate voters.

“That turnaround is unrivaled in my time,” said Jonathan Tasini, a progressive strategist and Sanders supporter.

Sanders underperformed across the map and never recovered, leaving his supporters reeling at the swift reversal of fortunes.

Many of Sanders’s supporters wanted him to stay in the race and fight until the end, believing he would keep Biden in check and that he had earned the right to leave on his own terms.

In his exit speech, Sanders said that he felt the pain of his disappointed supporters.

But he said his continued presence in the race was a distraction from the health crisis racking the nation.

“I know that there may be some in our movement who disagree with this decision, who would like us to fight on to the last ballot cast at the Democratic convention,” Sanders said.

“I understand that decision. But as I see the crisis gripping the nation, exacerbated by a president unwilling to 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,” he added.

The South Carolina primary will be remembered as the turning point in the election.

House Majority Whip James Clyburn (D-S.C.), a former chairman of the Congressional Black Caucus, endorsed Biden, and black voters broke by huge margins in favor of the former vice president.

Former South Bend, Ind., Mayor Pete Buttigieg Pete ButtigiegBogeymen of the far left deserve a place in any Biden administration Overnight Defense: Woodward book causes new firestorm | Book says Trump lashed out at generals, told Woodward about secret weapons system | US withdrawing thousands of troops from Iraq A socially and environmentally just way to fight climate change MORE and Sen. Amy Klobuchar Amy KlobucharEPA delivers win for ethanol industry angered by waivers to refiners It's time for newspapers to stop endorsing presidential candidates Biden marks anniversary of the Violence Against Women Act, knocks Trump and McConnell MORE (D-Minn.) dropped out of the race shortly after South Carolina and backed Biden.

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Some on the left hold bitter feelings against the mainstream Democrats who rallied to defeat Sanders. There is also deep frustration with the media’s treatment of him.

But in the end, Sanders could not compete in a one-on-one against Biden, who proved to be a more popular candidate than Hillary Clinton Hillary Diane Rodham ClintonHillicon Valley: FBI chief says Russia is trying to interfere in election to undermine Biden | Treasury Dept. sanctions Iranian government-backed hackers The Hill's Campaign Report: Arizona shifts towards Biden | Biden prepares for drive-in town hall | New Biden ad targets Latino voters FBI chief says Russia is trying to interfere in election to undermine Biden MORE, whom Sanders challenged in 2016.

Not all of Sanders’s supporters are despairing.

Many expressed optimism about the future of the progressive movement now that Sanders had laid the groundwork to incorporate young people and working-class voters into the Democratic Party.

Sanders’s grassroots donor model reinvented campaign fundraising, and his proposals on "Medicare for All," living wages for workers, environmental reforms and free college tuition will have lasting impacts on Democratic policy debates.

“It would have been absolutely thrilling to see him take the oath of office,” said Tasini.

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“But I’m extremely encouraged and optimistic about the long-term future of the movement, both in terms of the number of people involved, the types of candidates we’re seeing run for office, excitement among the activist base, the ability to raise enormous sums of money for our candidates and the state of the issues we’re promoting. The pace of which we’ve advanced between 2015 and now has accelerated at warp speed, and in just four years we’ve put the progressive movement on a much stronger path,” he added.

For many on the left, the work now turns to ensuring Biden engages with Sanders’s supporters and does not compromise when it comes to enacting a liberal agenda.

“Biden has his work cut out for him in terms of winning support from progressives,” said Neil Sroka, a progressive strategist for the liberal group Democracy for America. “I do think progressives will ultimately turn out to make sure Trump is defeated in November, but it will require work to ensure that the millions of young people getting involved this year for the first time don’t drop out of the process.”