The Democratic race is moving south as voters in South Carolina have their say on who should be the party's White House nominee.
We're still a long way off knowing who will take on Donald Trump in the autumn, but after the three contests so far - Iowa, New Hampshire and Nevada - Senator Bernie Sanders leads, trailed by ex-mayor Pete Buttigieg.
Former Vice-President Joe Biden is looking at the state - and its African-American voters in particular - as a life raft for his floundering campaign.
Whoever wins on Saturday evening will gain both delegates and crucial momentum into the spring.
Here's what to look out for in the next leg of the 2020 election.
A quick refresher
It's primary season in America.
Starting with the Iowa caucuses in February, all the way to a Puerto Rico primary in June, the party nominee will be chosen through a series of contests in every US state and territory.
In all likelihood, the Republican nominee will be Trump. But for Democrats, it's still a toss-up. The remaining eight candidates are hoping to win an unbeatable majority of 1,990 delegates nationwide - awarded through caucuses and primaries - to guarantee their nomination.
South Carolina's voters will cast the final votes before so-called Super Tuesday next week, when a third of delegates will be chosen.
One person to watch
Analysis by the BBC's Anthony Zurcher
Joe Biden was once the clear front-runner for the Democratic presidential nomination. A few weeks ago, after a string of defeats, his campaign teetered on the edge of electoral abyss. Now the former vice-president is scrambling his way back into contention.
While the campaign is reluctant to admit it, the South Carolina primary - the first in the southern US, with an electorate that better reflects the diversity of the Democratic Party than Iowa or New Hampshire - is make or break for Biden.
"If you send me out of South Carolina with a victory," Biden said on Wednesday, "there will be no stopping us."
That remains to be seen. This much is certain, however.
Win, and Biden's campaign lives to compete in the state contests ahead. Lose, and it's just about time to lower the curtain on his half-century career in American politics.
20 delegatesBernie Sanders' current lead over his 2020 rivals
As it stands, Senator Bernie Sanders sits at the top of the Democratic pile in terms of delegate count.
He is 20 delegates ahead of his nearest competitor, Pete Buttigieg, who trails with 25, followed by Joe Biden's 15.
With months to go and thousands of delegates left unclaimed, a mere 20-delegate difference may not seem like much. But the small sum belies Sanders' increasingly formidable lead.
The 78-year-old solidified his frontrunner status last week after claiming a decisive victory in Nevada.
And, importantly, entrance polls show he won a commanding 53% of the Hispanic vote. This could mean there's more good news coming for Sanders as he moves to votes in both California and Texas next week - both diverse and delegate-heavy states.
What does this all-white candidate field get wrong about African American voters?
One thing to watch for
South Carolina Republicans won't get a primary on Saturday, but some will vote anyway.
It is one of 15 states with an open primary, meaning that Democrats, Republicans and independent voters alike can all cast their ballots in the Democratic contest.
And some conservative groups, like Operation Chaos, are taking advantage - encouraging Republicans to cast a ballot for Bernie Sanders.
The "primary purpose" is to protest open primaries, said Christopher Sullivan, an organiser for Operation Chaos.
But, as Sullivan sees it, there's also a fringe benefit: disrupt the Democratic race.
So why Sanders?
Senator Sanders is the candidate that the Democratic establishment "least wants", Sullivan says. "I think it will prolong the pain of the Democratic primary for the Democrats."
The actual influence of this so-called "party raiding" on results is thought to be nominal, but will likely be a talking point as Saturday's primary unfolds.
Nicholas Judd, 18, Aiken, South Carolina
I have yet to choose a candidate that I support, or even a party as I am a moderate, centrist independent. I will wait to hear from both nominees in the lead up to the general election to decide whether to vote for the main party nominees or to write in a candidate that I can morally support. It seems that there are too many candidates that I do not like instead of one that I can actually support.
What's at stake in the election, I believe, is for America to find its voice as a unified state. Regardless of the outcome, I believe the biggest and most important task for the country is to find healing and unity from the tumultuous past four years.
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