Joe Biden will win North Carolina and Virginia, CBS News projects, as 14 states vote to pick a Democratic White House candidate on Super Tuesday.
Bernie Sanders, meanwhile, is projected to win his home state of Vermont, according to CBS, the BBC’s US partner.
Mr Biden, a moderate Democrat, and Mr Sanders, a left-winger, offer starkly different visions for America’s future.
They lead the race to anoint a Democrat who will face President Donald Trump, a Republican, in November’s election.
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.
Former New York Mayor Michael Bloomberg, who has spent half a billion dollars of his own money, and Massachusetts Senator Elizabeth Warren are on the ballot, too.
California, Texas, Massachusetts, Tennessee, Alabama, Arkansas, Oklahoma, Minnesota, Vermont, Colorado, Utah and Maine, along with American Samoa, also cast ballots on the biggest day of the race so far.
Super Tuesday offers an electoral bonanza of 1,344 of the 1,991 delegates needed to clinch the Democratic White House nomination under America’s quirky political system.
If no presumptive nominee emerges after the dust settles from Tuesday, the fight could grind away until the party convention in Milwaukee, Wisconsin, in July.