What is a caucus?

An arcane and, frankly, weird system of grassroots democracy. The word comes from the Algonquin tribe of Native Americans, meaning a “gathering of tribal chiefs”, and the custom dates back to the 18th century when political power-brokers met to discuss candidates. Most states now hold primary elections with registered Republicans and Democrats voting in a secret ballot to pick presidential nominees — but not Iowa

Have the Iowa caucuses always mattered?

No. It was only in 1972 after rules were changed to open up the caucuses to all party supporters and Iowa moved the date forward to January so that it was “first in the nation” that the rest of America took notice. In that year George McGovern took a surprise second place, giving him the momentum needed to take the Democratic nomination. In 1976 the little-known Jimmy Carter came storming out of the cornfields to win the White House, assuring Iowa’s place at the centre of the political universe every four years. In 2004 it was John Kerry’s victory and Howard Dean’s “scream” over his third-place finish that transformed the Democratic race

Does Iowa usually get it right?

It probably thinks so, but the winner in the caucus does not always go on to take his or her party’s nomination. In 1980 George Bush Sr narrowly beat Ronald Reagan but lost out in the end. In 1988 Richard Gephardt won the Democratic caucus only for Michael Dukakis to become the nominee. In 1992 the caucuses were bypassed because an Iowa senator, Tom Harkin, was standing. But Iowa usually influences what happens afterwards, providing momentum for trailing candidates and halting the progress of many a front-runner

Why can’t other states go first?

They tried this year but Iowa and New Hampshire (which stages the first primary next week) responded by moving their dates to the earliest yet, causing problems for campaigns because of a clash with the Christmas and new year holidays. For instance, many student supporters of Barack Obama are still at their family homes outside Iowa and no one knows if they will turn up

So what happens on Thursday night?

At 7pm about 200,000 people (7 per cent of Iowa’s population and 0.066 per cent of the US population) will go to the community halls, school gyms, churches and homes where 1,784 neighbourhood precinct meetings are being staged for both the Democrats and Republicans. They will spend an hour or so discussing the candidates and then they will vote

That sounds simple enough — how do they vote?

Ah, this is where the brain begins to ache. The rules are different for the two parties. Republicans write the name of their favoured candidate down on a sheet of paper — and every vote counts. The Democrats have an extraordinary system, a cross between a badly segregated football match and a school dance, in which caucus-goers gather in clusters according to which candidate they support. Any group with less than 15 per cent (or in some precincts 25 per cent) of those present in the room is deemed “non-viable” and its support can be realigned

Realigned — is that painful?

Blood is rarely spilt. But the second-choice votes from supporters of no-hope candidates, such as Chris Dodd, will probably decide who wins Iowa for the Democrats on Thursday night. Small groups sometimes stick together, desperately trying to persuade uncommitted caucus-goers to join them so that they can reach the threshold. More often, bigger groups try to cajole or entice supporters of the “non-viable” over to them. It is all supposed to be very neighbourly (this is, after all, the Midwest), but arguments do happen: legend has it that two neighbours in Fort Dodge have refused to speak to each other since the 1980 caucus

How many people are at each caucus?

It varies massively. In 2004 there were four precincts where no one turned up, and several more where the turnout was just one person. In the bigger towns, however, there may be hundreds. A big turnout does not mean that that precinct has more influence on the result

Why?

Precincts are awarded a certain number of delegates, based on a mathematical formula that takes into account votes cast in recent elections. Every viable candidate gets at least one delegate, with the remainder distributed in rough proportion to the size of the group. But it is quite possible to end up with the same number of delegates as a much smaller group

What happens to these delegates?

They go to one of 99 different county conventions, which, in turn, choose delegates to the Iowa state convention where delegates are elected to go to the national party convention in August

This is horribly complicated — when will we know who has won?

Don’t worry. The system is opaque but the delegate count will be clear by late Thursday

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