Leftist US Senator Bernie Sanders suspended his presidential campaign on Wednesday, clearing the way for rival Joe Biden to secure the Democratic nomination and challenge Donald Trump in November.

The feisty 78-year-old democratic socialist shook up the 2020 race with his relentless pursuit of "economic justice" for all Americans and a demand for universal health care.

But he acknowledged his campaign fell short, as party voters determined Biden would be a stronger candidate to go up against Trump in the general election.

"I have concluded that this battle for the Democratic nomination will not be successful," Sanders told supporters in a livestream from his home where he has remained for the bulk of the coronavirus pandemic that put all in person campaigning on hold.

"Vice president Biden will be the nominee," he said, adding that he congratulated Biden, "a very decent man, who I will work with to move our progressive ideas forward".

Sanders, who challenged Hillary Clinton for the party's nomination in 2016, mounted a formidable 2020 bid. He raised astonishing amounts of money from record numbers of donors, becoming the frontrunner early this year and earning the most votes in the first three state-wide contests.

But he was eclipsed by a surging Biden, who won the vast majority of remaining primaries and now holds a commanding lead in the all-important race for delegates who choose the nominee.

Sanders brought his liberal ideological platform -- including a call for universal health care and a $15 hourly minimum wage -- into the mainstream.

"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.

Several US lawmakers have come out in support of his policies, and Biden has shifted leftward to incorporate some Sanders positions, although he does not support Sanders's Medicare for All plan.

Biden seeks party unity

Biden immediately hailed Sanders as "a good man, a great leader, and one of the most powerful voices for change in our country". He urged Sanders' supporters to join his campaign, which already has the backing of nearly all other ex-rivals in the race including senators Kamala Harris and Amy Klobuchar, and former Indiana mayor Pete Buttigieg.

"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 in a statement to Sanders and his supporters. "I hope you will join us. You are more than welcome. You're needed."

Trump made his own pitch for Sanders's supporters, though in a more abrasive tone.

"Bernie Sanders is OUT!" the president posted on Twitter, saying Senator Elizabeth Warren helped split the liberal vote by remaining in the race during key contests, thereby helping Biden, the Democratic establishment's clear choice for the nomination. "The Bernie people should come to the Republican Party, TRADE!"

Trump won the 2016 election with help from disaffected working-class voters who believed they were being left behind by politicians in Washington.

Sanders also appeals to those voters.

On Wednesday he warned the coronavirus was causing an "economic meltdown" that has resulted in millions of lost jobs and a scramble for many to regain access to health insurance.

Sanders said he would not remain in an unwinnable campaign that would "interfere" with important anti-coronavirus work while the country is gripped by crisis.

But he stressed he would remain on the ballot and seek to gain as many delegates as possible in order to "exert significant influence" over the direction of the party.