Under the US election system, the nationwide popular vote does not determine the winner.
The candidates are both actually competing to win at least 270 electoral votes in state-by-state contests.
Those votes are apportioned to states based on a mix of population and representation in Congress.
In 41 states plus the District of Columbia, the results are fairly predictable.
States such as Texas and Alabama are almost certain to support Mitt Romney, while California and New York are among those favouring Barack Obama.
Mr Obama and Mr Romney are therefore battling for support from the thin slice of undecided voters in nine "swing states", where voters do not reliably vote for Democrats or Republicans.
These states include Florida (worth 29 electoral votes), Virginia (13), North Carolina (15), New Hampshire (four), Ohio (18), Wisconsin (ten), Iowa (six), Colorado (nine) and Nevada (six).
The two biggest swing state prizes are Ohio and Florida, but Ohio is seen as the most important, and it has received massive attention from both candidates in the final days.
Polls show Mr Obama holds a slight lead in a majority of the battleground contests where the outcome of the vote is likely to be determined.
In Florida, polls show the candidates running even, but Mr Obama has a slight lead in Ohio. Both states are crucial for Mr Romney.
If Mr Obama wins in Florida, Mr Romney would need to sweep all the other swing states, including Ohio.
If Mr Romney wins in Florida, but loses Ohio, Mr Obama would still be just 15 electoral votes shy of victory.
That would leave him with several paths to victory. For example, he could win by taking Nevada (six) and Wisconsin (ten), two states where polls show him leading.
Because of those stakes, many analysts see Ohio as the pivotal state on election night.
No Republican presidential candidate has ever won the White House race without also carrying the Midwestern state.
An Associated Press tally shows Mr Obama ahead in states with 237 electoral votes; Mr Romney leads in states with 191.
If that holds, Mr Obama must win in enough swing states to accumulate 33 more electoral votes. Mr Romney needs 79 more.
Nebraska and Maine have a proportional system of awarding electors.