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.
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.