Super Tuesday marked a nationwide success for former vice president Joe Biden as he was projected to win in nine states out of 15 contests, securing more than 300 delegates in the Democratic Party primary.
A total of 1,991 delegates are needed to secure the nomination and the successful candidate will run against US President Donald Trump in the November presidential election.
Results from California and Maine have yet to be announced, with the race being too tight to call.
But Mr Biden surged ahead and is projected to win in states including Virginia, North Carolina, Alabama, Tennessee, Oklahoma, Minnesota and Arkansas.
He is also expected to stage an upset in Massachusetts, the home state of Elizabeth Warren, marking a significant loss for her campaign. Early results showed Ms Warren in danger of finishing third in her state.
North Carolina, Virginia and Massachusetts would signal big wins for Mr Biden, as together they amount to 300 delegates.
In addition, the key state of Texas, which represents 228 delegates, was projected to go to Mr Biden after a tight race against Bernie Sanders.
On the eve of Super Tuesday, a number of endorsements were made in Texas by former candidates Pete Buttigieg, Amy Klobuchar and Beto O'Rourke, who all pledged their support to Mr Biden.
He was also endorsed by Senator Tim Kaine, the 2016 Democratic vice presidential candidate, and former Virginia governor Terry McCaullife.
Progressive candidate Mr Sanders is expected to win in his home state of Vermont, in addition to Colorado and Utah.
Mr Sanders’ biggest victory is expected to be in California, with AP projecting he would win the state soon after polls closed.
However, the final results from California were delayed due to long voting lines and the state's mail-in voting policy.
California awards 415 delegates, the largest number available on Super Tuesday. In the build-up to the vote, Mr Sanders invested heavily in California, setting up 22 campaign offices in the state, compared to Mr Biden’s single California office.
Billionaire candidate and latecomer to the race Michael Bloomberg, who spent $500 million (Dh1.83bn) of
his own fortune on campaign advertising has only been projected to win American Samoa.
NBC reported that Mr Bloomberg will be reassessing his campaign tomorrow.
Mr Biden, the biggest winner of the night, gave a fired-up speech from Los Angeles celebrating his victories, and trying to unify the party.
“Our campaign reflects the diversity of this party and this nation and that’s how it should be,” he said.
Two “anti-dairy” protesters interrupted the speech as Mr Biden was speaking.
But Mr Biden’s closest rival Bernie Sanders was nowhere near conceding the race. In a defiant speech from Vermont, Mr Sanders blasted Mr Biden's record and insisted he was the better candidate to defeat Mr Trump.
He criticised Mr Biden's position on trade and for voting for the Iraq war.
"One of us in this race led the opposition to the war in Iraq – you're looking at him. Another candidate voted for the war in Iraq."
The states voting were Alabama, Arkansas, California, Colorado, Maine, Massachusetts, Minnesota, North Carolina, Oklahoma, Tennessee, Texas, Utah, Vermont and Virginia, as well as US territory American Samoa.
Together, they account for 1,357 delegates, and a candidate is required to secure the support of 1,991 delegates to win the nomination.
Voter turnout in Texas was hit by coronavirus fears while power cuts left some California voters in the dark.
Meanwhile, a powerful tornado disrupted voting across stretches of Tennessee. Polling stations in three counties in the state suffered extreme damage and voters were redirected to cast their ballot at other locations within the state.
A judge extended voting hours in the Nashville area after four Democratic candidates sued to keep Super Tuesday polls open after the tornado damage.
Before Tuesday evening, Mr Sanders led the delegate count with 56, while Mr Biden was in the second place with 48.
Only a week ago, the former vice president’s campaign appeared to have run out of steam.
But he was buoyed by a big win in South Carolina on Saturday.
Meanwhile, Mr Trump took to Twitter on Tuesday night to mock rival Mr Bloomberg for eating doughnuts and licking his fingers on the campaign trail.