Caucuses and Primaries Explained

The first test of public opinion in the 2008 White House race is the Iowa caucus on January 3, followed five days later by the New Hampshire primary.

These are traditional testing grounds, often producing surprises, propelling marginal candidates to front runner status and ruining favorites.

How does the Iowa caucus work?

Democratic and Republican voters will gather at 1,784 precincts, often in a school building or community center, to choose the presidential candidates. Some places will see a large turnout, possibly a few no one at all.

In theory, only people registered as Democrats or Republicans can vote but it is possible to register on the day. Each campaign team relies on someone good at maths to help judge the support.

How does the primary work?

The primary is the more popular route to selecting candidates: a secret ballot, using polling booths throughout the state. In some states, it is closed, with only those registered as Democrats or Republicans able to vote; in others, any citizen can vote.

Democratic state primaries use proportional voting; the number of delegates sent to the party convention later in the year is relative to the candidates' percentage of the vote. The Republicans allow proportional or winner-takes-all ballots.

What happens next?

The states then send their delegates to the party conventions in 2008. Democrats will send their elected delegates to their convention in -Denver next August; the Republicans in September, at Minneapolis St Paul.

Most states have between 30 - 150 delegates and the candidate who wins a majority of delegates nationwide gets the nomination. In reality, it is likely to be decided long before the conventions. The candidates could be chosen on Super Tuesday, on February 5, when about 20 states, including California and New York, hold their primaries.