Joe Biden has won six of the 14 states that voted to pick a Democratic White House candidate on Super Tuesday, in a remarkable rebound for his campaign.
The former US vice-president has swept Oklahoma, Arkansas, Alabama, Tennessee, North Carolina and Virginia.
Left-wing Senator Bernie Sanders is expected to win Colorado and his home state of Vermont.
They lead the race to anoint a Democrat who will face President Donald Trump, a Republican, in November’s election.
Mr Biden, a moderate Democrat, and Mr Sanders, a left-winger, offer starkly different visions for America’s future.
US media are also projecting that Mr Biden has won Minnesota.
CBS News, the BBC’s US partner, projects Massachusetts is also leaning Mr Biden’s way, a heavy blow to his rival and the liberal state’s own Senator, Elizabeth Warren.
Buoyed by last-minute endorsements from former rivals who dropped out of the race, Mr Biden is hoping to blunt the momentum of Mr Sanders, who was the frontrunner nationally on the eve of the vote.
Mr Biden’s under-staffed and under-funded campaign has been resurgent since his commanding win in South Carolina’s primary at the weekend. It was the first time the 77-year-old had won any contest in three White House runs.
Exit poll data suggests Mr Biden has won large majorities of African-American voters, a crucial bloc for the Democratic party.
Mr Biden also appears to have won convincingly among the type of suburban voters who pollsters say have been turning away from President Donald Trump.
With all precincts reporting, Mr Biden won about 53% of the vote in Virginia, leaving Mr Sanders a distant second on 23%.
Virginia and North Carolina are so important because they are key swing states in the 2020 election.
All eyes now are on the biggest voting state of California and the second-biggest, Texas.
Mr Sanders is hoping to do well in the Golden State, a liberal behemoth, while exit poll data suggests he won 45% of the Hispanic vote in Texas, compared to 24% for Mr Biden.
Also appearing on a presidential ballot for the first time after skipping all four contests last month was Michael Bloomberg.
The former New York mayor, who has spent more than half a billion dollars of his own money, is expected to win the US territory of American Samoa.
Mr Biden aims to consolidate the centrist Democratic vote by barging aside Mr Bloomberg, one of the world’s richest men.
But the billionaire wildcard lowered expectations for his performance ahead of Tuesday, indicating he plans to fight all the way to the party convention in Milwaukee, Wisconsin, in July.
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Super Tuesday offers an electoral bonanza of more than 1,300 of the 1,991 delegates needed to clinch the Democratic White House nomination under America’s quirky political system.
Amy Klobuchar and Pete Buttigieg, who were also competing for centrist Democrats, ended their campaigns this week and endorsed Mr Biden.
The Sanders team have said the double exit was part of an all-out mobilisation by the party establishment and big money donors to freeze out the Vermont senator.
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.
Mr Biden presents himself as an electable pragmatist who can bring incremental change, whereas Mr Sanders has promised a revolution.
Mr Sanders’ detractors say he is too much of a radical firebrand to win over the swing voters needed to win 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.
Mr Biden’s critics say his campaign is uninspiring, and he brings too much political baggage from his lifelong career as a Washington insider.
Mr Trump, meanwhile, could not resist taking a dig at Mr Bloomberg.
The president tweeted a video of his rival New York billionaire eating a donut, commenting: “Mini Mike, don’t lick your dirty fingers. Both unsanitary and dangerous to others and yourself!”