Joe Biden has won eight of the 14 states that voted to pick a Democratic White House candidate on Super Tuesday, a remarkable rebound for his campaign.
The ex-US vice-president has swept Massachusetts, Minnesota, Oklahoma, Arkansas, Alabama, Tennessee, North Carolina and Virginia.
But Bernie Sanders is being projected to win the biggest prize of the night, California, and three other states.
They lead the race to face Republican President Donald Trump in November.
Former New York mayor Michael Bloomberg spent more than half a billion dollars of his own money on his campaign, but did not win a single state.
Super Tuesday awards more than 1,300 of the 1,991 delegates needed to clinch the Democratic White House nomination in July. As things stand, Mr. Biden has 396 delegates and Mr. Sanders 314.
Virginia and North Carolina are crucial because they are key swing states in the 2020 election.
Exit poll data across the board suggests Mr. Biden has attracted large majorities of African-American voters, a crucial bloc for the Democratic party.
Mr. Biden also appears to have won among the type of suburban voters who pollsters say have been turning away from the current US president.
“We are very much alive,” Mr. Biden told a crowd in Los Angeles. “Make no mistake about it, this campaign will send Donald Trump packing.”
If Mr. Sanders does win California, as the Associated Press news agency projects, he will pick up the lion’s share of the whopping 415 delegates that the Golden State sends to the party convention.
The left-wing senator also won his home state of Vermont, along with Colorado and Utah.
All eyes now are on the second biggest voting state of the night, Texas.
Mr. Sanders had been heavily favoured to win the Lone Star State, but it is currently too close to call between him and Mr. Biden.
Partial results from Maine’s primary also show Mr. Sanders and Mr. Biden neck and neck.
In a victory speech, Mr. Sanders lambasted Mr. Trump, but also took a shot at Mr. Biden.
“We’re taking on the political establishment,” he said. “You cannot beat Trump with the same-old, same-old kind of politics.”
Senator Elizabeth Warren, once the frontrunner in the race, suffered a humiliating defeat to Mr. Biden in her home state of Massachusetts.
Mr. Bloomberg could only pick up a consolation win in the tiny US territory of American Samoa.
He has, however, garnered enough votes in California and Texas to pick up delegates, according to incomplete results.
Mr. Bloomberg was appearing on a presidential ballot for the first time after skipping all four contests last month.
The billionaire wildcard had lowered expectations for his performance ahead of Tuesday. But on the eve of the vote, he indicated he plans to fight all the way to the party convention in Milwaukee, Wisconsin, in July.
Mr. Trump could not resist taking repeated digs at his rival New York billionaire after his disappointing results.
The Democratic party is at a crossroads as its voters decide which candidate has the best chance of denying Mr. Trump a second term in office this autumn.
Once a crowded field of more than two dozen, celebrated for its women and candidates of colour, the Democratic contest now increasingly looks like a two-man race between two white male septuagenarians.
Mr. Biden, a moderate who served under President Barack Obama, and Mr. Sanders, a liberal firebrand, offer starkly different visions for America’s future.
The former vice-president presents himself as an electable pragmatist who will bring incremental change and restore “decency” after the Trump presidency.
But critics say his campaign is uninspiring, and that he brings too much political baggage from his lifelong career as a Washington insider.
Mr. Sanders’ detractors say a self-described democratic socialist cannot win over the swing voters needed to capture the White House.
The 78-year-old is planning to transform the American economy with a multi-trillion dollar, higher taxation blueprint on everything from healthcare to education.
BBC