The Iowa caucus is the first test for US Democratic presidential candidates. The results are hard to predict for two reasons: voters’ indecision, and a caucus system that means they may have to go with their second choice.

The Midwestern state of Iowa is home to some 3 million people, yet it has outsized importance because its caucus comes first in the presidential primary cycle.

An individual caucus is essentially a small local meeting in a school gym or community centre – known as a precinct – where neighbours and strangers show their support for a particular candidate by standing in groups and trying to persuade others to join them.