As the Iowa caucus looms Denis Stauntonin Des Moines explains the primary system's intricacies
Next Thursday, tens of thousands of Iowans will step out into a freezing, mid-western winter night and make their way to a local school, a courthouse or a neighbour's home to choose the next American president.
Only about one in 10 eligible voters in the state are expected to attend the Republican and Democratic caucuses, but Iowa's choices will have a dramatic impact on the race, propelling some candidates to success in the primaries that follow and marking the end of the road for others.
After a year of campaigning in the most extended presidential election in US history, the candidates are entering an intense series of sudden-death contests that could determine the nominees for both parties by the first week in February.
The nominees will not, in fact, be chosen until the Democrats and Republicans hold their national conventions in late August and early September and the caucuses and primaries will choose delegates to attend those conventions.
When Americans register to vote, they can choose a party affiliation or declare themselves to be Independents. In the primaries, registered supporters of each party vote for the candidate they want to represent their party in the presidential election. A few states, including New Hampshire, allow registered Independents to vote in either the Democratic or Republican primaries - although they can't vote in both.
A handful of states, like Iowa, hold caucuses - meetings of party supporters that require voters to turn up at an appointed place and time and show their support for their candidate. Supporters of each candidate form groups in different parts of the room to be counted. In Iowa, the Republican caucuses are simple straw polls, but the Democrats allow a second round of voting in which supporters of candidates who have too little support to be viable (usually below 15 per cent) can realign with other groups.
Until 1968, most states didn't hold presidential primaries at all, and those that were held were not binding on delegates and were seen mainly as a tool to show the electability of particular candidates. At the 1968 Democratic national convention in Chicago, amid rioting outside and bitter rows inside, the nomination went to Hubert Humphrey - whose backers had won just 2 per cent of the popular vote in the primaries.
After Chicago, the Democrats reformed their nominating system to ensure that delegates are apportioned according to the presidential preference expressed by voters in the primaries. There are a few loopholes but most states divide their delegates at the convention according to the primary results. This means that a candidate can lose a primary but still pick up delegates for the convention.
The Republicans allow states to run "winner-take-all" primaries, which can help a front-runner with a strong organisational and financial base and proved a huge advantage to George W Bush in 2000. Unlike the Democrats, the Republicans do not require state delegations to follow the preference expressed by primary voters.
In 2008, the primary calendar has been compressed so that most states - including the biggest - will hold primaries within the first five weeks of the year. On February 5th - known as Super-Duper Tuesday - more than 20 states will vote, among them California and New York.
This "front-loading" of the primary system, which was motivated by the desire of big states to increase their influence over the process, could have the perverse effect of making the traditionally early-voting states - notably Iowa, New Hampshire and South Carolina - more important than ever. Among the leading candidates in both parties, only former New York mayor Rudy Giuliani is ignoring the early contests in the hope of delivering a knock-out blow to his rivals on February 5th.
Although the system has become more democratic in recent years, neither party has been willing to hand uncontrolled power to the people.
Voting alongside delegates chosen by the states, the Democrats have super-delegates - 842 unpledged delegates including governors, members of congress, former presidents and members of the Democratic National Committee. Altogether, they account for nearly 20 per cent of the votes at the national convention and in a close race, they could determine the outcome.
The 2008 presidential election is the first since 1928 without an incumbent president or vice-president running for either party's nomination. The Republican field is unusually fluid, with at least five candidates - Giuliani, Mike Huckabee, John McCain, Mitt Romney and Fred Thompson - who have a plausible chance of winning.
Among the Democratic frontrunners, both Hillary Clinton and Barack Obama have raised more than $80 million, enough to remain in the race to the very end. Although most commentators expect the nominees to be determined by early February, each race also has the potential to produce, for the first time in decades, a brokered convention in late summer.