The caucus is the oldest method of choosing delegates.
Registered members of a political party in a town, city or county meet, usually in a place in the community, such as a school or church, to elect delegates to be put forward for presidential elections.
Although once more common, caucuses these days only take place in a few states, notably Iowa, Nevada and Alaska.
While Republican voters cast secret ballots, Democrats vote publicly in a slightly more unusual process, in which their candidates need the support of 15% of voters. Should a candidate fall below this threshold, the voter can choose another candidate.
Primaries, on the other hand, are much more common, conducted in 34 US states. They are similar to elections in the UK, with voters casting their ballot at polls.
There are several types of primary, varying between both states and parties; closed, in which only a party’s registered members can vote; semi-closed where members and unaffiliated voters can participate; open, where any registered voter can participate; and semi-open, where any registered voter may take part but must request a party’s specific ballot.
What happens next?
Once the primaries and caucuses are complete, the delegates go on to the National Conventions, which take place a week apart in July, Republican on 18-21 and Democratic on 25-28, when the presidential candidates are announced.
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