US Election: How Primaries And Caucuses Work "Caucuses And Primaries Explained"

Adam Boulton, political editor
For much of election year, 2012 will look like a one-horse race. 
That is because one side, the Democratic Party, already have their candidate - the incumbent President Barack Obama.
Mr Obama will be out campaigning across the US but for much of the year he will be waiting to see who the Republicans nominate to challenge him - a matter which may well not be resolved until the summer.
The Republicans are wasting no time getting their state-by-state nomination process going.
As other states vie for the early publicity, Iowa has been forced to hold its traditional opening caucuses on the first Tuesday of the year.
The first primary, in an equally chilly New Hampshire, takes place seven days later.
Iowa and New Hampshire really witness pavement politics in the flesh.
The person who may end up the most powerful politician in the world spends the day gladhanding relatively tiny groups of people, perhaps repeating an almost identical "stump speech" a dozen times.
Meanwhile, an equally important and costly "ground war" to get out the candidate's vote is waged by volunteers and paid professionals, on foot, online and by phone.
It is a truly impressive demonstration of democracy in action. 
Even British reporters can get a more intimate look at the candidates than they would, say, in a typical UK parliamentary by-election.
Each state has its own idiosyncratic rules for selection.
Caucuses involve groups of people meeting in small communities across the state. 
They discuss the candidates and then form clusters for each one: the man or woman who gets the most warm bodies wins.
In primaries, votes are cast. Sometimes these are closed to registered party supporters only, or they may be open to any interested voters.
In this off-year for their primaries, some Democrats will be tempted to take part mischievously in open Republican primaries.
The Republicans have already staged 13 televised debates and there are still eight officially declared candidates in the race for the nomination - Michele Bachmann, Newt Gingrich, Jon Huntsman, Ron Paul, Rick Perry, Buddy Roemer, Mitt Romney, and Rick Santorum.
Two viable candidates - Herman Cain and Tim Pawlenty - have already fallen by the wayside, while many other leading Republicans - most notably Jeb Bush, Sarah Palin and Mike Huckabee - have decided this is not going to be their year.
So the main function of the early caucuses and primaries is to shake out the field to a more manageable handful of viable runners.
Outsiders will weigh the cost of going on fighting against dropping out, perhaps in favour of a front-runner who might even put them on the ticket as a vice-presidential candidate or give them a job in government if elected.