Fourteen US states from coast to coast will vote on Tuesday in the largest Democratic primary for a nominee to take on President Donald Trump on November 3.
The Super Tuesday vote could play out in different ways but all promise to be a turning point for the race between Bernie Sanders, Elizabeth Warren, Joe Biden and Mike Bloomberg.
Minnesota senator Amy Klobuchar on Monday night announced that she would be leaving the race to endorse centrist Mr Biden.
Former mayor of South Bend, Indiana, Pete Buttigieg and billionaire Tom Steyer ended their campaigns at the weekend, after a major win for former vice president Mr Biden in South Carolina.
Mr Buttigieg was the first openly gay candidate to come this far in the race.
On Tuesday, Alabama, American Samoa (which is a US territory), Arkansas, California, Colorado, Maine, Massachusetts, Minnesota, North Carolina, Oklahoma, Tennessee, Texas, Utah, Vermont and Virginia will decide the fate of 1,357 delegates.
A winning nominee needs 1,991 delegates to take the nomination and so far only 4 per cent of the delegates have been chosen.
Mr Sanders leads the delegate count at the moment with 56, while Mr Biden is second with 48.
But with a growing divide between the leftist base of the Democratic Party and the moderate camp, different scenarios could be at play in the Super Tuesday primaries.
For the current frontrunner, Mr Sanders, a landslide in California, which has 416 delegates, and a good performance in Texas and Massachusetts could boost his lead for the convention in July.
He tweeted last week that he is unstoppable:
But his momentum has been halted after an almost 20 per cent loss to Mr Biden in South Carolina.
With African-American support, the former Vice President is hoping to replicate that win in many Super Tuesday states, especially those with a large presence of people of colour.
These include North Carolina, Oklahoma, Tennessee and Virginia.
Mr Biden is also banking on his relationship with the most popular democratic figure, former president Barack Obama.
Mr Obama called Mr Biden after the South Carolina win but is staying on the sidelines and will not be endorsing a candidate before the convention, Bloomberg News reported:
The Biden campaign has had key endorsements in Virginia from Senator Tim Kaine and former state governor Terry McCauliffe, but is still running behind in California.
Mr Buttigieg’s endorsement of Mr Biden is likely but it is unclear if it will happen soon or after Super Tuesday, CNN reported.
The other progressive Senator in the race, Elizabeth Warren, who was the frontrunner nationally in October, is struggling to make key wins in the Super Tuesday votes.
At the moment, Ms Warren is polling second or third in California, Texas and Virginia but is behind Mr Sanders in her own state of Massachusetts.
She sharpened her attack on Mr Sanders over the weekend, saying that his “30-year track record shows he consistently calls for things he fails to get done".
Ms Warren's campaign is insisting that no candidate will be able to accumulate the 1,991 delegates needed to grab the nomination before July.
But the Super Tuesday vote will be a defining moment for the campaign of Mr Bloomberg, who will for the first time be on the ballot after skipping the early states.
The former mayor of New York has spent more than $400 million (Dh1.46 billion) in advertising to make his case in these states and is hoping for a strong effect.
But Mr Bloomberg's poor performance in the debates and attacks from other candidates have weakened his campaign in the past two weeks.
Whether the Super Tuesday states produce a presumptive nominee or a four-person race, the Democratic battle will be dramatically reshaped by Wednesday morning.