Vermont senator Bernie Sanders has dropped out of the presidential race, paving the way for his Democratic rival, former vice-president Joe Biden, to take on Republican President Donald Trump in November.

Wednesday's announcement prompted calls for the Democratic Party to unite as it pivots to taking on Mr Trump, even as the President immediately sought to play up intra-party divisions in his response to the news.

"We are now some 300 delegates behind (former) vice-president Biden, and the path towards victory is virtually impossible," said Mr Sanders, 78, who announced the suspension of his second run for the presidency in a video streamed live on his website.

"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, a day after Wisconsin controversially went ahead with in-person voting in its primary election despite a statewide stay-home order.

Congratulating Mr Biden, Mr Sanders called him the party's nominee and promised to work with him to advance his movement's progressive ideas.

But Mr Sanders said he will remain on the ballot so he can continue gathering delegates to the Democratic convention, as leverage to influence the party's platform. "Then together, standing united, we will go forward to defeat Donald Trump, the most dangerous president in modern American history," he said.

The senator's announcement capped a primary cycle full of twists and turns.

Mr Sanders, a Democratic Socialist, nonetheless briefly became the party's front runner after several early wins in February, amid a wide field of candidates who fractured the race.

The establishment then rallied around a flagging Mr Biden, denying Mr Sanders wins in a series of crucial states and highlighting his lack of support from African-American and white suburban voters. Both blocs would have been crucial to victory.

The Covid-19 crisis also upended the primary calendar, forcing the postponement of primary elections in 15 states, most until early June.

The delay threatened to drag out the fight between Mr Sanders and Mr Biden, giving the eventual winner less time to consolidate support while national attention moved on from the race.

Mr Sanders has been credited with pushing progressive policies - from single-payer health insurance to a US$15 (S$21) per hour minimum wage and free college tuition - to the forefront of the national political conversation over the course of his decades-long political career.

"It was not long ago that people considered these ideas radical and fringe. Today they are mainstream ideas, and many of them are already being implemented in cities and states across the country," he said in his speech.

Mr Biden, in a lengthy statement on Wednesday, said: "Senator Sanders and his supporters have changed the dialogue in America. Issues which had been given little attention - or little hope of ever passing - are now at the centre of the political debate."

He added: "While Bernie and I may not agree on how we might get there, we agree on the ultimate goal for these issues and many more."

Mr Biden should now be on a mission to attract Sanders supporters who might otherwise not vote, American University historian Ibram X. Kendi tweeted. "Like it's on Biden to attract White swing voters, it's on Biden to attract other swing voters: younger Sanders supporters who swing from voting Democrat to not voting, or voting 3rd party."

On Twitter, Mr Trump sought to deepen the intra-party rift as he blamed Mr Sanders' loss on his fellow presidential hopeful, Massachusetts senator Elizabeth Warren, whom he suggested without evidence had diverted liberal votes from Mr Sanders.

Mr Trump also sought to inflame the resentment of segments of Sanders supporters, who alleged a Democratic establishment plot against the senator after he lost to former secretary of state Hillary Clinton in the 2016 campaign.

Said Mr Trump: "This ended just like the Democrats and the DNC (Democratic National Committee) wanted, same as the Crooked Hillary fiasco. The Bernie people should come to the Republican Party, TRADE!"