Senator Bernie Sanders has ended his US presidential campaign after disappointing primary results, leaving Joe Biden as the likely Democratic nominee.

The Vermont senator’s announcement makes former vice president Mr Biden the presumptive Democratic nominee to challenge President Donald Trump in November.

“The path toward victory is virtually impossible,” Mr Sanders told supporters as he congratulated Mr Biden.

The former vice president is “a very decent man whom I will work with to move our progressive ideas forward”, he said.

Mr Biden credited Mr Sanders for creating “a movement” and appealed to his progressive supporters.

“I see you, I hear you, and I understand the urgency of what it is we have to get done in this country,” Mr Biden said.

“I hope you will join us. You are more than welcome. You’re needed.”

Mr Sanders initially exceeded sky-high expectations about his ability to recreate the magic of his 2016 presidential bid, and even overcame a heart attack last October on the campaign trail.

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 78-year-old senator 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 would be a major presidential contender this cycle, especially as the race’s oldest candidate.

Mr Sanders, though, used strong polling and solid fundraising — collected almost entirely from small donations made online — to more than quiet early doubters.

Mr 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.

But a crucial endorsement of Mr Biden by influential South Carolina Representative Jim Clyburn, and a subsequent, larger-than-expected victory in South Carolina, propelled the former vice president into Super Tuesday, when he won 10 of 14 states.

Election 2020 Bernie Sanders
Joe Biden preparing for a Democratic presidential primary debate with Bernie Sanders, who has announced he is suspending his campaign (Evan Vucci/AP)

In a matter of days, his top former Democratic rivals lined up and announced their endorsement of Mr Biden.

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 Mr Sanders.

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

The coronavirus outbreak essentially froze the campaign, preventing Mr Sanders from holding the large rallies that had become his trademark and shifting the primary calendar. It became increasingly unclear where he could notch a victory that would help him regain ground against Mr Biden.

Though he will not be the nominee, Mr Sanders was a key architect of many of the social policies that dominated the Democratic primary, including a “Medicare for All” universal, government-funded health care plan, tuition-free public college, a 15 US dollar minimum wage and sweeping efforts to fight climate change under the “Green New Deal”.

He relished the fact that his ideas — viewed as radical four years ago — had become part of the political mainstream by the next election cycle, as Democratic politics lurched to the left in the Trump era.

Mr Sanders began the 2020 race by arguing that he was the most electable Democrat against Mr Trump.

He said his working-class appeal could help Democrats win back key states that Mr Trump won in 2016, including Michigan, Wisconsin and Pennsylvania.

But as the race wore on, the senator reverted to his 2016 roots, repeatedly stressing that he backs a “political revolution” from the bottom up under the slogan “Not me. Us.”

Mr Sanders made clear that while he is exiting the campaign, he will keep pushing for progressive principles.

“Please stay in this fight with me,” he told supporters. “The struggle continues.”