Mitt Romney could become only the fourth presidential candidate to win the popular vote but lose the race for the White House, an outcome that would renew scrutiny of America's quirky and controversial electoral college system.
About 10 swing states will decide the winner, with the president currently holding a slender lead in those he needs to win to secure the 270 electoral college votes required for victory.
It is entirely possible therefore that Obama could win the electoral college and the White House while receiving fewer votes overall than Romney.
That scenario would be a repeat of 2000, though with the parties reversed, when the Democrat Al Gore won the popular vote, but George W Bush narrowly won the electoral college by five votes after being awarded Florida in contentious circumstances.
Such a division between the popular vote and the electoral college has happened only three times in US history, with the previous occasion before 2000 coming in 1888. Romney is neck and neck in the national polling with Barack Obama, so he has a 50-50 chance of winning a simple majority of votes. But the president leads in most of the swing states, giving him an easier path to the 270 electoral votes needed.
The college is not a place but a process, though the system does involve real people, the electors, who are distributed among the 50 states and the District of Columbia according to the size of each state's Congressional delegation.
The sizes vary greatly: California is the largest with 55, while the likes of sparsely-populated Alaska and Montana have only three. As decreed by the constitution, each state's electors will meet in December - on the first Monday after the second Wednesday - in their state capitals and the cast their votes for president and vice-president. The electors are typically local political leaders chosen by their parties.
In 30 states they are required to reflect the popular will, but in others they have been known to deviate and choose another candidate or make a protest abstention vote.
Most states have a "winner-take-all" system that awards all electors to the winning presidential candidate, although Maine and Nebraska each have a variation of proportional representation.
The system was designed by the founders with two goals. The college electors were supposed to act as a safety net in case the masses chose a candidate of whom the elite disapproved.
The adoption of winner-takes-all was a bargain struck between northern and southern states. It was intended to strengthen the political clout of the South, which had lower voting populations but high economic power.
Supporters argue that changing it to a direct vote for the president would concentrate too much power in the hands of urban populations to the detriment of rural, more sparsely populated states.
Critics say that even in an ordinary election, millions of voters in 40 or so non-swing states easily feel disenfranchised, because the system heightens the importance of the swing states so dramatically.
Noises are made about reform, but no serious effort has ever made headway.