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.
Mr Biden took North Carolina, Massachusetts, Minnesota, Oklahoma, Arkansas, Alabama, Tennessee and Virginia and is poised to win Texas.
But Bernie Sanders is projected to win California – the biggest prize of the night – as well as three other states.
They lead the race to face republican President Donald trump in November.
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.
Exit poll data across the board suggests former US vice-president 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.