US election race: How it all works

With 300 million Americans spread over 50 states, trying to organise an election for the most powerful job in the western world is no easy task.

The race for the White House is a straightforward contest between incumbent Democrat Barack Obama and his Republican challenger Mitt Romney.

But the system that will choose the winner by tomorrow morning is complex to say the least.

While in Britain, MPs are elected by a simple "first past the post" vote, America relies entirely on so-called electoral colleges.

In the controversial 2000 election, George W Bush lost the "popular vote". He got 50,456,002 votes from individuals while his Democrat rival Al Gore got 50,999,897.

The reason Bush went on to be president was because he had won more than 270 votes through the electoral college system.

This time around, Obama is predicted to overcome the Romney challenge – but only just.

The reason for that prediction is that the president is ahead in polls in the state of Ohio, one of 11 so-called battleground states that will determine who becomes the latest President of the United States.

Ohio is important because it comes with a sizeable 18 electoral votes. No President in recent electoral history has been voted in without first winning Ohio.

In the earliest days of the United States, the electoral college was a way of choosing one person to be president over lots of different states of various sizes and populations.

Each state gets a certain number of votes based on its size. So while Ohio is given 18 votes, a small state like Rhode Island only gets four.

Meanwhile, heavily populated California gets 55 and Florida 29.

Across all the 50 states there are 538 votes up for grabs under the electoral college.

The first candidate to pass 270 will be declared the winner. That could be good news for anyone hoping to stay up late tonight and watch the action unfold. The first results will start to come from areas like Indiana and Kentucky at around 11pm UK time. But polling stations will not even close in Alaska until 5am. Thanks to the electoral college system though, the race will probably be won long before the world finds out which way Alaska voted.

If one of the candidates soars ahead it might be possible to predict the result at around 1.30am.

Traditionally, the result can be called in favour of one or the other by around 4am, once California and Washington's votes are counted. It is customary for the loser to call the winner, congratulate him and admit defeat.

Of course things could still be too close to call, as they were in 2000, and delays could result in recounts and even legal challenges.

If after all the recounts there is a dead heat with 269 electoral college votes each, the House of Representatives, which is the equivalent of our House of Commons, would vote to choose the next president.

If Mitt Romney does beat Barack Obama tomorrow morning, he will not become president straight away. In Britain a defeated Prime Minister moves out of Downing Street straight away – unless the result is a hung Parliament like it was in 2010.

In America there is a two-month "transition". Obama would stay in charge until January, when Romney would be sworn in.

If Obama wins he will only be allowed to serve one more four-year term according to a rule written into the American constitution in 1947.

Until that point, it had only been an unwritten rule that presidents serve no more than two consecutive terms.

The longest serving president was Franklin D Roosevelt, who was elected for a fourth term in the final months of the Second World War but died shortly after the war ended.

This has been an especially close election campaign which has seen the candidates almost neck and neck.

The arrival of superstorm Sandy last week, which left a trail of death and destruction along the north-east of America, has thrown the campaign into disarray and meant it was all to play for in the final days.

For Mitt Romney it has been a struggle to maintain a high profile without appearing to be using the personal tragedies of so many for political gain.

The Republican had been planning to flood TV stations with last-minute adverts, having held back a significant chunk of campaign funding.

Last week, in the wake of the superstorm he held a "storm relief event" in Kettering in Ohio where he collected food for the victims, but did it in the same arena as his previously planned rally and featuring the same line-up of celebrities.

For Barack Obama, his response to the devastation will be as important in people's decisions today as anything he achieved, or failed to achieve, in the past four years.