As Americans head to the voting booths, campaign teams for both candidates have been furiously working the phones to urge supporters to cast their ballots.
Opinion polls show incumbent president Barack Obama and his Republican rival Mitt Romney are in a virtual dead heat, although according to the latest polls Obama has a slight advantage in several vital swing states that could give him the 270 electoral votes needed to win the state-by-state contest. With the result in the balance, voter turn-out may prove decisive.
The outcome looks likely to be settled in just a handful of key states. US states are attributed electoral votes according to their population from California, the most populous, with 55 electoral votes and states with smaller populations having just three.
See our interactive map of electoral states
Many analysts predict Ohio, with its 18 electoral votes, will be the crucial swing state.
Ohio is home to more than 800,000 automotive jobs and industry watchers credit the president with saving many of them when he agreed to bail out General Motors and Chrysler three years ago. Romney opposed the plan.
Earlier Obama made a surprise visit to a campaign office in his home city of Chicago. He expressed confidence he would win re-election and congratulated Romney on a “spirited campaign”.
Mitt Romney, the Republican presidential nominee, and his wife Ann, cast their ballots at a voting station not far from their Belmont, Massachusetts home. Romney will watch the election proceedings at an event in Boston.
Vice President Joe Biden voted in his home state of Delaware. “I’m feeling pretty good,” Biden said when asked if he had any prediction. The 69-year-old former Senator, who twice ran unsuccessfully for the White House before becoming Obama’s running mate, has not ruled out another run in 2016.
Republican Vice-Presidential candidate Paul Ryan cast his vote in the town of Janesville, Wisconsin, where he grew up. Ryan is considered charismatic and youthful, an image some Republicans feel the Romney campaign is currently lacking.
At least 120 million people are expected to vote. Their decision will set the country’s course for the next four years on spending, taxes, healthcare and foreign policy, challenges like the rise of China and Iran’s nuclear ambitions.
As well as choosing a new commander in chief, voters will decide who will sit in the Senate and the House of Representatives.
See below: electing the US president