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Former US vice-president Joe Biden has been handed a huge boost in the race to challenge Donald Trump in November's election.
He is set for a large victory in South Carolina's primary, where voters have been picking who they want to be the Democratic nominee for the election.
Left-winger Bernie Sanders, who is second in South Carolina, is likely to remain in the lead overall.
Another 14 states vote on Super Tuesday this week.
By the end of Super Tuesday, it could become much clearer who the nominee will be. South Carolina is only the fourth state to have voted so far in the months-long primary season.
This is Mr Biden's first ever victory in a primary in what is his third run for US president. In a victory speech, he said: "Just days ago the press and the pundits had declared this candidacy dead. Now, thanks to all of you – the heart of the Democratic Party – we just won, and we've won big."
What happened in South Carolina?
With three-quarters of the votes counted, Mr Biden has 50% of the share, ahead of Mr Sanders and billionaire hedge-fund manager Tom Steyer. It is Mr Steyer's best performance of the primaries, but he will now end his campaign, leaving seven candidates in the running.
Mr Biden had been pinning his hopes on a strong result in the southern state, after performing poorly in Iowa, New Hampshire and Nevada.
He regularly cited his strong support among African-Americans, and polls suggest an endorsement by influential black congressman James Clyburn played a significant part in how people voted.
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He also appears to have performed well among voters over the age of 45, an exit poll by Edison Media Research suggests.
Victory will hand the 77-year-old a boost ahead of the biggest day of voting on Tuesday, and allow him to make the case that he is the best-placed moderate Democrat to take on Mr Sanders.
Candidates who have won more than 15% of the vote in South Carolina will be awarded delegates, who will then go to the party's convention in July to support their Democratic candidate.
It looks like Mr Biden could claim the vast majority of the state's 54 delegates, with Mr Sanders the only other candidate to cross the 15% threshold.
In his speech, Mr Biden attacked Mr Sanders – an independent senator running for president as a Democrat – without naming him.
"Democrats want a nominee who is a Democrat," he said. "A life-long Democrat. A proud Democrat. An Obama-Biden Democrat. We have the option of winning big or losing big."
What happens next?
This Tuesday is Super Tuesday, the most important date in the race to pick the nominee.
Democrats in 14 states will vote (as well as American Samoa and Democrats Abroad). A massive 1,357 delegates will be distributed – almost a third of all those available through the entire primary season, and the two most populous states, California and Texas, will be voting.
The entire picture could change in one day. Or we could see Mr Sanders cement his lead as the front-runner – and even extend into a near-unbeatable lead, as seems possible.
This will also be the first time that New York billionaire Michael Bloomberg will be on the ballot – the performance of moderates like Mr Biden on Super Tuesday will be determined to some degree by that of Mr Bloomberg.
We've pulled together a guide on what to look out for in each state on Super Tuesday.
