Joe Biden and Bernie Sanders are the only major candidates to remain in the Democratic primary race after an initially crowded field gradually winnowed down. Elizabeth Warren was the last major candidate to drop out after a disappointing result on Super Tuesday.

The primaries and caucuses are a series of contests, in all 50 US states plus Washington DC and US territories, by which the party selects its presidential nominee. In these votes, the goal for the Democratic candidates is to amass popular support that translates to a number of pledged delegates.

To secure the nomination, a candidate needs the support of 1,991 pledged delegates. If no candidate has reached this number by the party’s national convention in Milwaukee in July, it is known as a contested or brokered convention. The nominee is then chosen in a more complex voting system that also involves unpledged delegates – known as ‘superdelegates’ – who can vote their conscience.

The nominee will eventually face Trump in the general election on 3 November.