The midwestern state is the first to vote in the presidential primary race. So what are caucuses, and how do they work? Here’s your guide to the night

The Iowa caucuses take place on Monday 3 February, kicking off the long process of nominating a Democratic presidential candidate who will eventually take on Donald Trump in November’s US election.

The primary race is made up of a series of contests called primaries and caucuses that take place in all 50 states plus Washington DC and outlying territories, by which the parties select their presidential nominee from the candidates who are running.

The goal in these contests is for candidates is to amass support from voters that translates into a majority of delegates, whose job it is to nominate a presidential candidate at the party conventions in July and August.

When it comes to choosing a presidential candidate, Iowa traditionally goes first. Though Iowa has relatively few delegates, it is highly influential because it gives Americans their first chance to see what support the candidates have, and a win could provide a vital boost in momentum, as it did for Barack Obama in 2008.

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Democratic and Republican caucuses will take place on Monday evening, but because Trump does not face any serious competition from his challengers for the nomination, all eyes will be on the Democratic contenders this year.

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What is a caucus?

Most states hold primary elections, which are run by the state, and resemble general elections, in which voters go to a polling place, mail in their ballots or otherwise vote remotely. But a handful of states hold caucuses, hours-long voting meetings run by the party. Republican caucuses are simpler, and Democratic caucuses are more complicated.

It makes for a drawn out night, as results are tallied from more than 1,600 different locations across Iowa – all 55,000 square miles of it.

When can we expect the results?

Patience will be key. Iowans who are eligible to take part will have to be at their caucus venue (usually a school, church or community center) in their local precinct at 7pm CT (8pm ET) in most cases – a tough ask itself given the frigid Iowa winters.

Once there, it’s not a matter of ticking a box and going home. They will spend up to two hours casting their preference for presidential candidate.

As for when we can expect to know the winner: it’s hard to say. In 2016 the Democratic race between Bernie Sanders and Hillary Clinton was so close that the results were announced the next day (Clinton won by a razor-thin majority). On the Republican side, Ted Cruz beat Donald Trump, and was declared winner before midnight.

How do the Democratic caucuses work?

In most other states, primary voting works similarly to voting in a regular election. Voters turn up at a polling station and cast their vote privately. They usually have all day to vote, and don’t have to tell anyone who they voted for. Not so in Iowa.

It isn’t a secret ballot. In that venue, a representative for each candidate will be standing in a certain area of the room.

Voters have to go and stand in the area where their candidate is represented. So Joe Biden supporters would be in one corner, Elizabeth Warren’s in another, and Bernie Sanders supporters in another. Everyone is tallied.

For the Democratic caucuses, in particular, second choices are important. If a particular candidate does not attract 15% of total voters present, they are not seen as “viable” and are taken off the ballot, leaving their voters free to throw their weight behind another candidate. For example, a Pete Buttigieg supporter could sidle over to the Warren area.

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Once the bartering for supporters is over, the votes are totted up, and state delegates are awarded to each candidate proportionately. That, in turn, determines how many national convention delegates each candidate receives.

The candidate with the most state delegate equivalents “wins” Iowa. Except…

There could be confusion.

For the first time, the Democratic party plans to release three sets of numbers from different stages in the process. It will release the raw tally of votes each candidate received taken before the 15% cut off, and also release the tally of votes candidates received once supporters have realigned. Finally, the party will publish the STEs, which represents the number of delegates assigned to each candidate.

However, the “winner” of the Iowa caucuses is still based on the number of state delegate equivalents each candidate receives.

How do the Republican caucuses work?

With all the attention being paid to the Democratic side, it is easy to forget that there is a Republican primary happening, too.

Trump is facing challenges from the former Massachusetts governor Bill Weld and Joe Walsh, who served one term in the House of Representatives, among less well-known candidates.

It’s extremely unlikely that Weld, or Walsh, or the others (according to the Federal Election Commission 153 people are running for president as Republican) will win, but at least they will navigate a less a complex system than the Democrats.

In the Iowa Republican caucus, people still have to physically show up at a location. They then cast a straightforward vote, secretly. The votes are tallied up, and the delegates are dished out in the same manner. In the Republican caucuses, there is no 15% threshold, meaning candidates with relatively little support could still pick up delegates.