But despite early disappointments Sanders was still expected to rack up large numbers of delegates in by far the biggest states of the night, Texas and California, where polls were due to close at 0400 GMT.
With delegates in 14 states up for grabs, Biden was projected to win in Virginia, North Carolina, Alabama, Oklahoma, Tennessee, Arkansas and even Minnesota -- a state where Sanders had been expected to win handily.
The 78-year-old Sanders, a self-described democratic socialist with a fervent voter base, who aims to reshape America's economy, was projected to win as expected in his home state of Vermont, Colorado and Utah.
In a defiant speech, Sanders tore into Trump, calling him "the most dangerous president in the history of this country." But he also tore into Biden for having voted in favor of the invasion of Iraq and painting him as tarnished by billionaire contributors.
"We're taking on the political establishment," he said. "You cannot beat Trump with the same-old, same-old kind of politics."
But for Biden, 77, the indications were that he was on for a big night in his bid to bring American politics back to the center after four years of Trump's rightwing populism.
"We are very much alive," he told a crowd in Los Angeles. "Make no mistake about it, this campaign will send Donald Trump packing."
A key takeaway from Biden's lengthening list of wins was the strong support for the former vice president of Barack Obama from African Americans -- a vital piece in any Democratic presidential candidate's coalition.
Former New York mayor Michael Bloomberg, 78, and Massachusetts Senator Elizabeth Warren were the other big names on the ballot.
But despite spending record amounts of his own money on advertising, billionaire media entrepreneur Bloomberg appeared to be headed for an ugly night -- despite a consolation win in tiny American Samoa in the Pacific.
Stop the Sanders campaign
The 14 nominating contests across the country gave the dwindling field of Democratic hopefuls a giant potential haul of delegates in their marathon struggle to win the nomination -- and begin campaigning in earnest against Trump.
Many in the Democratic Party are desperate to stop Sanders' strong push to win that delegate race, saying the senator will be destroyed in a general election where Trump has signaled he will brand him a socialist bent on ending the American way of life.
Biden was practically counted out after a stumbling early campaign, but began to rebound with a landslide win in South Carolina last Saturday.
That was followed by the coordinated decisions of two other moderate candidates -- Pete Buttigieg and Amy Klobuchar -- to withdraw and endorse their former rival.
Bloomberg faces multiple calls from within the Democratic Party also to get out of the race and not split the centrist vote.
In a statement, Bloomberg's campaign insisted he was still competing. But it added: "Our number one priority remains defeating Donald Trump in November."
Commenting on Bloomberg's feeble Virginia results, veteran political analyst Larry Sabato asked: "A hint to say bye-bye?"
Sanders fans convinced
Biden is making his third bid for the White House after failed runs in 1988 and 2008. He argues that he can bring a return to "decency" after the tumultuous, scandal-plagued Trump era.
Sanders' fans are convinced that only he can take on Trump, who also defied his party's establishment and more moderate wing four years ago to claim a surprise victory against Democratic heavyweight Hillary Clinton.
"We need energy. We need excitement. I think our campaign is that campaign," said Sanders.
Supporter Jamison Hanning, a 45-year-old plastics industry technician, said he was "pretty confident" despite the Biden pushback.
"I mean it is just people in the establishment being afraid of things being shaken up," he said.
Sanders went into Super Tuesday hoping to gain a near insurmountable delegate lead, perhaps delivering a knock-out blow well ahead of the party convention in Milwaukee in July.
A total of 1,357 delegates were at stake on Tuesday -- a third of the nationwide total. A candidate needs 1,991 delegates to win the nomination outright.
Democrats will also be looking for turnout and other signs of enthusiasm in a country deeply divided by Trump.
Certainly huge turnout in Virginia indicated excitement ahead of November, with nearly double the voters as in the Democratic primary there four years ago.
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