Vermont Sen. Bernie Sanders, who saw his once-strong lead in the Democratic primary evaporate as the party's establishment lined swiftly up behind rival Joe Biden, ended his presidential bid on Wednesday — an acknowledgement that the former vice-president is too far ahead for him to have any reasonable hope of catching up.

Sanders advised his campaign staff of his decision in a morning conference call and then gave a virtual news conference to media and supporters, bowing out in a speech that contained references to Nelson Mandela and Martin Luther King, and addressed the COVID-19 pandemic.

"As I see the crisis gripping the nation, exacerbated by President Trump, I cannot in good conscience continue to mount a campaign that cannot win," he said.

While falling short of the nomination a second time, the progressive Sanders said it was clear his followers had helped win "the ideological struggle" over the future of the Democratic Party.

"Together we have transformed American consciousness as to what kind of nation we can become and have taken this country a major step forward in the never-ending struggle for economic justice," he said.

WATCH | Sanders addresses supporters after bowing out of Democratic presidential race:

Despite saying 'the future of this country is with our ideas,' Vermont Sen. Bernie Sanders has ended his campaign to become the Democratic nominee for U.S. president. 0:48

Sanders suffered a heart attack in late 2019 but put himself in position to compete for the nomination in what had been a crowded field after strong results in Iowa, New Hampshire and Nevada.

But he saw his hopes for the nomination slip away in March after a series of wins for Biden, now the presumptive nominee to challenge Donald Trump, who Sanders characterized as "the most dangerous president in modern American history."

In a series of tweets, Biden congratulated Sanders on the race he ran, praising him for a "movement that is as powerful today as it was yesterday."

In a separate statement, Biden praised Sanders and promised his ideas would be incorporated into his own White House run.

"I see you, I hear you, and I understand the urgency of what it is we have to get done in this country," Biden said to Sanders's supporters. "I hope you will join us. You are more than welcome. You're needed."

And to Bernie’s supporters: I know that I need to earn your votes. And I know that might take time. But I want you to know that I see you, I hear you, and I understand the urgency of this moment. I hope you'll join us. You're more than welcome: You're needed. —@JoeBiden

'Electability' fears about Sanders

Sanders had faced pressure to bow out both due to the results and the pandemic, which has upended the Democratic race and also raised questions about voting for the general election, which is scheduled for Nov. 3.

Sanders reacted angrily to court rulings that saw the Wisconsin primary proceed on Tuesday, saying his campaign wouldn't engage in traditional get-out-the-vote efforts in the state due to health concerns over the coronavirus.

Sanders congratulated Biden and called him a "decent man."

Sanders and Biden skipped the traditional handshake for the final debate, which was moved to Washington from Phoenix on March 15 because of the coronavirus pandemic. (Evan Vucci/The Associated Press)

He also said on Wednesday, however, that his name would remain on the ballot and he would continue to amass delegates as a "practical matter" for a Democratic convention that is now scheduled for August. Several Democratic primaries have been pushed back to May and June.

Sanders initially exceeded sky-high expectations about his ability to recreate the magic of his 2016 presidential bid. But he found himself unable to convert unwavering support from progressives into a viable path to the nomination amid "electability" fears fuelled by questions about whether his democratic socialist ideology would be palatable to general election voters.

The senator, who is an independent but caucuses as a Democrat, began his latest White House bid facing questions about whether he could win back the supporters who chose him four years ago as an insurgent alternative to the party establishment's choice, Hillary Clinton. Despite winning 22 states in 2016, there were no guarantees he'd be a major presidential contender this cycle.

'The future of the country is with our ideas'

Sanders, though, used strong polling and solid fundraising — collected almost entirely from small donations made online — to more than quiet early doubters. Like the first time, he attracted widespread support from young voters and was able to make new inroads within the Hispanic community, even as his appeal with African Americans remained small.

Sanders amassed the most votes in Iowa and New Hampshire, which opened primary voting, and cruised to an easy victory in Nevada — seemingly leaving him well positioned to sprint to the Democratic nomination while a deeply crowded and divided field of alternatives sunk around him.

After a strong showing in the Iowa, New Hampshire and Nevada primaries and caucuses, Sanders's fortunes began to turn after Biden's resounding victory in South Carolina and on Super Tuesday. (Patrick Semansky/The Associated Press)

In a matter of days after a big Super Tuesday showing for Biden in early March, his top former Democratic rivals lined up and announced their endorsement of his rival. The former vice-president's campaign had appeared on the brink of collapse after New Hampshire but found new life as the rest of the party's more moderate establishment coalesced around him as an alternative to Sanders.

Things only got worse the following week when Sanders lost Michigan, where he had campaigned hard and upset Clinton in 2016. He was also beaten in Missouri, Mississippi and Idaho the same night and the results were so decisive that Sanders headed to Vermont without speaking to the media.

Thank you Bernie - for doing your best to fight for all of us, from the beginning, for your entire life.<br><br>Thank you for fighting hard, lonely fights in true devotion to a people’s movement in the United States.<br><br>Thank you for your leadership, mentorship, and example. We love you. <a href="https://t.co/CqErUDlsvX">https://t.co/CqErUDlsvX</a> <a href="https://t.co/33x7hm5ErH">pic.twitter.com/33x7hm5ErH</a> —@AOC

Sanders had scheduled a rally in Ohio but cancelled it amid fears about the spread of coronavirus — and the outbreak kept him home as his campaign appeared unsure of its next move. The senator addressed reporters the following day, but also sounded like a candidate who already knew he'd been beaten.

"While our campaign has won the ideological debate, we are losing the debate over electability," Sanders said on March 18.

As if with that address, Sanders on Wednesday pointed to "sometimes overwhelming" support for his candidacy among younger voters.

"The future of this country is with our ideas," he said.