Tuesday is the biggest day of the US primary calendar, when 14 states from the Atlantic to the Pacific vote for their Democratic presidential nominee.
These include the nation’s two most populous states, California and Texas, and nearly one third of all the delegates at July’s Democratic National Convention are up for grabs.
Here are some key questions ahead of Super Tuesday:
– Can Sanders recapture the narrative?
Vermont senator Bernie Sanders has promised that he could substantially expand the electorate beyond traditional Democratic voters, but that has not happened in the first four contests.
Super Tuesday represents his biggest chance to prove his case. He is far better financed and organised than former vice president Joe Biden, who trounced Mr Sanders in South Carolina on Saturday.
With five major candidates running, it has been impossible for anyone to claim a clear majority, but Mr Sanders’ durable base has given him a crucial plurality of the vote, and, more importantly, a small lead among delegates.
Mr Sanders needs as many delegates as possible because his opponents argue the convention itself should decide the nominee, should no-one secure a clear majority.
He argues that the candidate with the most votes should get the nomination — an easier case to make if he comes into the Democratic National Convention with a clear plurality.
Tuesday represents Mr Sanders’ best chance of building a durable advantage in the race. Because the Democratic Party awards delegates proportionately, once someone racks up an advantage in the delegate count, it is difficult to catch them.
After Tuesday, the terrain shifts to states that are not as favourable to Mr Sanders.
– Can Biden emerge as the ‘stop Sanders’ candidate?
Joe Biden dramatically under-performed in Iowa and New Hampshire, part of a collapse among white voters that allowed Mr Sanders to build a lead. But Mr Biden regained his footing in South Carolina, propelled by the overwhelming support of black voters. That aided Mr Biden’s case that the candidate who prevails among these base voters will win the nomination.
Mr Biden’s campaign hopes that allows him to vastly over-perform his polls for Super Tuesday and consolidate the splintered anti-Sanders factions in the Democratic Party.
The hope in the former vice president’s camp is that it becomes effectively a two-person race after Tuesday, which may give him an advantage in upcoming states like Florida, Illinois and Pennsylvania.
– Which candidates can stay viable?
Fifteen per cent is the share of votes a candidate has to get to win delegates in primary elections. With five Democrats left in the race, there is a real threat that only one or two will actually grab delegates, with the rest splitting the remaining vote in the low teens.
The risk of that diminished with the exit of Pete Buttigieg from the race on Sunday night, and Minnesota senator Amy Klobuchar on Monday, but it certainly did not go away.
That would be a dream scenario for Mr Sanders, who in some states could grab a far greater proportion of available delegates than his vote share would represent. Some polls have shown it as a distinct possibility in delegate-rich California.
– Will Bloomberg’s big bet pay off?
Former New York Mayor Mike Bloomberg has bet more than half a billion dollars (£390 million) on Super Tuesday – soon we will know if this was money well spent.
Mr Bloomberg is a billionaire, and rather than competing in the first four primary states, he decided to spend a huge sum of money on advertisements and campaign organisations in the 14 states voting on Tuesday, as well as on other areas voting in the coming weeks.
But since he first appeared on the debate stage, his polls have plummeted, and now he runs the risk of falling into the sub-15% zone in a number of states.
Even if he does not, the question remains over whether Mr Bloomberg will fragment the anti-Sanders vote even further, which could pave the way for the self-proclaimed democratic socialist the billionaire said he specifically wanted to stop.
– How does the California count affect the race?
California is different. It is the biggest prize on the board on Tuesday with more than 400 delegates at stake.
The state also has an unusual voting system that counts all ballots cast on Tuesday, even if they were only put in a mail box that day. It can take weeks to tally the entire vote.
That could help or hurt Mr Sanders. His campaign has been investing heavily in the state, trying to encourage his supporters to send in early ballots. It may be that the early ballots heavily favour him, and we discover only in the coming weeks that California voters did not support the Vermont senator so overwhelmingly.
But Mr Sanders’ base includes young and Latino voters who are more likely to vote at the last minute, so the opposite could happen — his supporters may still turn in their ballots at the last minute, and the initial returns could look grimmer for him than the actual final results.
There is a good chance we will not know the final delegate disposition from the state until April. It remains to be seen how that uncertainty will affect the race in the weeks after Super Tuesday.