Joe Biden has won nine of the 14 states that voted to pick a Democratic White House candidate on Super Tuesday, a remarkable rebound for his campaign.
The former US vice-president overturned predictions to narrowly take the key state of Texas from his main challenger, Bernie Sanders.
They lead the race to face Republican President Donald Trump in November.
Former New York mayor Michael Bloomberg spent more than $500m of his own money on his campaign, but did not win a single state. And Senator Elizabeth Warren, once the frontrunner in the race, suffered a humiliating defeat to Mr Biden in her home state of Massachusetts.
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 402 delegates and Mr Sanders 314. However, results from California, which has 415 delegates, could affect the current standing.
How important is Biden’s performance?
Only last month, Mr Biden’s campaign was all but written off by some observers after he finished a poor fourth in the Iowa caucuses and fifth in the New Hampshire primary.
But on Tuesday, Mr Biden won Texas – the second biggest state with 228 delegates – along with Massachusetts, Minnesota, Oklahoma, Arkansas, Alabama, Tennessee, North Carolina and Virginia.
Maine, which has 24 delegates, is also yet to declare but partial results put Mr Biden slightly ahead of Mr Sanders.
Virginia and North Carolina are crucial because they are key swing states in the 2020 election.
Exit polls across the board suggested Mr Biden – who was vice-president to Barack Obama – attracted large majorities of African-American voters, a crucial bloc for the Democratic party.
Mr Biden, 77, 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.”
How is Sanders doing?
If Mr Sanders, 78, 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.
Mr Sanders had been heavily favoured to win Texas, but it was finally claimed by Mr Biden.
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.”
The next primaries take place on 10 March in Michigan, Washington state, Idaho, Mississippi, Missouri and North Dakota, with 352 delegates available.