29 November 2011 Last updated at 11:36 ET

Quick guide: US presidential elections

The road to the White House is long, complicated and expensive. BBC News explains the process.

The early stages

A politician with presidential ambition usually forms an exploratory committee to test the waters and raise money. This can begin up to two years before the election.

The US constitution requires only that a person be a "natural born citizen" of the US, at least 35 years old, and been a resident of the US for 14 years.

But not just anyone can credibly stand for election: In the last 70 years, every non-incumbent candidate has been either a sitting or former US senator, governor, vice-president or five-star general.

If the candidate attracts significant favourable notice from the news media, party officials, prospective campaign strategists, and donors, he or she formally declares the candidacy and launches campaigns in key early states.

Barack Obama's campaign began to coalesce in late 2006 and he formally declared he was running in February 2007.

Mitt Romney, meanwhile, has essentially been running for president for nearly five years.

Having failed to capture the Republican nomination in 2008, Mr Romney quietly maintained a behind-the-scenes campaign through the first two years of Mr Obama's term in the White House and formally declared his candidacy for the 2012 nomination in June.

The primaries

The primary season begins the January before the election and lasts until about June.

This is where candidates fight for the nominations of the two main parties, Republican and Democratic.

In the primaries and caucuses, voters in each of the 50 states, plus Washington DC and some US territories, select party delegates who in most cases pledge to support a particular candidate.

Some states use a caucus - a local meeting system - rather than primaries.

The party convention

The national party conventions are held in the late summer about two and a half months before the election.

There, the parties formally nominate the candidates chosen by party voters in the state primaries and caucuses.

Before or at the convention, the nominee picks a running-mate, sometimes from among the defeated rivals.

Because the nominee is almost always known before the start of the convention, the conventions are mostly geared toward promoting and celebrating the nominee and laying out the party's message and agenda.

The final lap

After the conventions, the candidates fully square up against each other on the campaign trail, over the airwaves and in widely-viewed debates.

Each spends millions of dollars on advertising and a major flurry of state-by-state campaigning.

This year, there will be three debates between the presidential candidates and one between the vice-presidential candidates, according to the Commission on Presidential Debates.

In the final weeks, the contenders typically concentrate their attention on big so-called "battleground" states, where the electorate is more or less evenly divided and hence the outcome is uncertain.

Watch for the candidates to blanket the airwaves with adverts in Florida, Ohio, Pennsylvania and Virginia.

The election

American presidential elections are always held on the first Tuesday after the first Monday in November. This year, it is 6 November.

Voters do not, technically, participate in the direct election of the president.

They choose "electors", who are pledged to one or another candidate, in a process known as the electoral college.

Each state has a certain number of electors to the college, based on the size of its population.

In almost every state, the winner of the popular vote gets all the electoral college votes in that state.

Because of this system, a candidate can take the White House without winning the popular vote, such as in the 2000 contest between George W Bush and Al Gore.

What's next?

In the days and weeks after the election - if the vote is decisive - the victor will assemble a cabinet and begin crafting a more thorough policy agenda.

Meanwhile, the departing "lame duck" president works to shape his legacy and begins packing up his belongings.

Under the US constitution, the president is inaugurated on 20 January of the year following the election.