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The American way of politics; How the system works

British politicians stand for office; Americans run. Britain has elections; America has "races".

Today there begins officially, with a small caucus in Louisiana, the greatest political race of them all, the lengthiest, costliest and most brutal democratic exercise in the world. The first big fences in this great steeplechase are the Iowa caucuses on 12 February and the New Hampshire primary on 20 February. The winning post is a mind-numbing nine months' away on 5 November.

Why so long? Why so costly?

With the rise of the primary system in the past 40 years, presidential elections have fallen into two parts. There is a rolling state-by-state campaign within the major parties from February until the conventions crown the Republican and Democratic nominees in the summer. Then there is a campaign between the party champions from September to November.

There are many criticisms of primaries. They lead to a period of bloody trench warfare within the party (the Democrats are exempt this time because Bill Clinton is unopposed); they force a prolonged campaign, which leaves media, public and politicians cynical and exhausted; they often defeat - or deter - the best candidates.

But there is also much to be said for primaries. In no other country does the public have a chance to select the party's standard-bearer as well as choose between the parties. In the US system, in which power is divided between the executive (the presidency) and legislature (Congress), recognised party leaders do not emerge from parliament.

The only alternative way to choose a party's candidate would be the old, discredited way: party bosses deciding in smoke-filled rooms and then manipulating the convention.

What is a primary? What is a caucus? A primary is a state-wide election in which all registered voters can take part. Caucuses take several forms but consist essentially of meetings throughout the state in which supporters vote by a show of hands or ballot. In some states, such as California, only those registered as Democrats can vote for Democrats and only Republicans can vote for Republicans. In other states, such as New Hampshire anyone can vote for anyone.

The rules, order and dates of primaries and caucuses vary from party to party, state to state and election to election. There has been one crucial change this time. California, the biggest prize of all, has moved its primary from June to late March. Others, not to be outdone, have also pushed forward. The effect will be to squeeze the real primary campaign into less than eight weeks. Primaries after California are likely to be meaningless, unless - as may just be possible - the Republican race becomes the tightest in recent history.

The lacklustre Republican field has led to speculation that someone else - General Colin Powell? - may be "drafted" by the party (i.e. picked by bosses) at the last moment. This used to be common. As recently as 1968, Hubert Humphrey won the Democratic nomination without entering a single primary.

The tighter rules now make this very difficult, if not quite impossible. On the first ballot at the convention, all delegates must vote according to the primary results in their state (or any deals made to reallocate the delegates of fallen candidates). Only on the second ballot are they free to vote for whichever candidate they like.

All of the above applies, of course, only to the major-party candidates. A well-heeled independent, such as Ross Perot, can join the race at any time and save his energy and money for the final campaign, which traditionally starts on Labour Day (2 September).

There is one other important fact to remember. US presidential elections are not a single, nation-wide battle for the popular vote. They are, in effect, mini-elections in each of the 50 states (and Washington DC). The candidate first past the post scoops all that state's delegates to the Electoral College (which, technically, picks the president later in the year). The number of Electoral College votes is allocated according to the size of the state. The effect of all this is to concentrate the final campaign in the 20 or so large, politically balanced states, rich in Electoral College votes, such as California or Illinois. No presidential candidate bothers going to Alaska, or even North Dakota. JOHN LICHFIELD