So far, Democrats in the relatively small states of Iowa, New Hampshire, Nevada and South Carolina have had to cast their vote on the candidate they want to see running for Donald Trump in this fall's presidential election. Together they have distributed a total of 155 delegates.

On Super Tuesday, 14 states and American Samoa (which do not have state status) are up for election. Together they will distribute 1,357 delegates among the candidates - hence the prefix “super”. This year's super Tuesday has become extra "super" after the giant state of California moved forward its primary election and is now part of the super Tuesday.

After Tuesday's election, about 38 percent of all delegates will be distributed, and are therefore considered a water divider in the nomination campaign. Some candidates may drop out if they do poorly, and it is possible that two or three candidates will definitely disengage the nomination.

Here are all states and territories that vote on Super Tuesday, from largest to smallest: California, Texas, North Carolina, Virginia, Massachusetts, Minnesota, Colorado, Tennessee, Alabama, Oklahoma, Arkansas, Utah, Maine, Vermont, American Samoa.

Here you can read more about how the primary elections work.

In both 2008 and 2016, the election on Super Tuesday was between just two candidates. But this year, at least four candidates are hoping to vote on Super Tuesday, after Pete Buttigieg and Amy Klobuchar late withdrawing their candidacies. Here they are ranked for support in national opinion polls.

• Bernie Sanders , a Vermont senator who attracted a great deal of attention when he challenged and almost won the nomination in 2016. Currently has the most delegates, leads opinion polls nationally and in most states.

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• Joe Biden , former vice president, ranks second in most opinion polls nationally and in the number of delegates, leading in many states. May be considered as the main challenger to Bernie Sanders right now.

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• Michael Bloomberg , former mayor of New York. A billionaire who published his candidacy so late that he did not have the opportunity to register in the first states, but only became eligible to vote on Super Tuesday. Bloomberg has spent hundreds of millions of dollars on advertising in Super Tuesday.

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• Elizabeth Warren , senator from Massachusetts. Has so far won very few delegates, but has made a strong showing in debates and hopes to take many delegates during Super Tuesday. There are four in national opinion polls, and it is currently difficult to see that she can win a majority of the delegates.

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The delegates act as voters' representatives and vote for their candidate at the party convention in Milwaukee, Wisconsin, in mid-July. To win the nomination, 1991 requires delegates - more than half. If no candidate reaches it, it is free for the candidates to co-negotiate with their delegates for a second vote. There, so-called super delegates may also vote, 771 delegates appointed by the party leadership and not by voters.

As there are an unusually large number of candidates still in the competition this year, it seems at this time that it is very likely that no candidate will come to the convention with enough votes to win in the first round of elections. That has not happened since 1984.

This year's nomination election is unusual. The candidate with the greatest support among voters, Bernie Sanders, lacks support in the party leadership and among the super delegates. The fact that Elizabeth Warren, who has many similar proposals as Sanders, still remains in the contest, diminishes his chances of getting his own majority of delegates, which opens for a second vote at the convention.

This is important because it may play into whether candidates such as Warren and Bloomberg are resigning if they do poorly on Super Tuesday. Do they choose to step aside to accommodate similar candidates, or do they stay until the end?

Given that the party's candidate may be nominated at the Convention itself in a second vote, there is still incentive for them to remain in the contest and co-operate for the nomination in a second vote, with the support of the party's super delegates.

If that meant that the candidate who received the most votes - Bernie Sanders, as it seems right now - does not receive the nomination, such a development could be highly controversial.