The first step on the long, exhausting path to the White House has begun, as voters across the US have begun to back candidates in the primary elections. Although George W Bush will not leave office until the 20 January 2009, the race to succeed him began in earnest on 4 January when candidates from both main parties – the Democrats and the Republicans - began competing for their party's nomination. For the first time since 1928, neither a sitting president nor vice president will be a candidate in the race, theoretically creating a wide-open fight for both nominations.

While the UK has a restricted General Election campaign period, the United States system is vastly longer and much more complicated. It consists of a primary election period, party conventions in the late summer, general election campaigning and then polling day, which is always held on the first Tuesday after the first Monday in November.

At the moment, candidates are in the midst of the primary election stage with most states holding state-wide votes, called primaries, to determine their preferred candidates from the two main parties. Other states use a slightly different procedure involving public meetings, called caucuses. In both cases the upshot is that the state will send a certain number of delegates committed to supporting a particular candidate, to the party convention in August or September 2008. The candidate with the most delegates wins the nomination. While the official primary election stage runs from January to June, it usually becomes apparent who the winning candidate will be much earlier on in the primary season. This year the winning candidates are expected to be known after Super Tuesday.

Super Tuesday is a Tuesday when a lot of states hold primaries or caucuses simultaneously. The practice began in the 1980s. In 2000, 16 states held primaries on 7 March, at which about 60% of all delegates were up for grabs. In 2004, Super Tuesday split in two, there was a Super Tuesday I on 3 February, followed by a Super Tuesday II on 2 March, when California, Ohio and New York all held their votes. This year, 24 states including California, New York and New Jersey have said they intend to hold primaries or caucuses on Tuesday, February 5. This makes it the day when most delegates backing a candidate can be won.

After the primary election period and once both parties have chosen their candidate, both parties hold their party conventions. The Democrats will hold their convention in Colorado in late August and The Republicans will hold theirs in Minneapolis in early September. After this, the presidential candidates will take part in TV debates on 26 September, and 7 and 15 October. There will also be furious campaigning in key battleground states before the presidential election is held on the 4 November.