After the longest, most expensive election campaign in US history, voting is underway to elect the 44th president.
Republican John McCain and Democrat Barack Obama returned to their home states of Arizona and Illinois to vote and hold final rallies.
Mr Obama is holding a steady lead in final opinion polls and record numbers of voters are expected to turn out.
In the first voting of the day on Tuesday, Mr Obama defeated his rival by 15 votes to six in Dixville Notch, New Hampshire. The town, which has a 60-year tradition of being first in the nation to vote, opened its polls at midnight, with turnout of 100%.
President George W Bush won there in 2004 on his way to re-election. Tuesday's vote was the first time the town had gone Democrat since 1968.
Another small New Hampshire town, Hart's Location, with a tradition of polls opening at midnight, has also gone for Mr Obama by 17 votes to 10.
Most other polls on the east coast will open at 6am EST.
The BBC reports a McCain victory would require a massive turnout of Republican voters, a surprise win in all the key states and more.
He would have to contradict all the polls, sway all the undecideds in his favour and prove once again he is a survivor. Mr McCain says he can do it, but the BBC reports it is looking increasingly unlikely.
On Monday, the rivals spent a hectic final day of campaigning criss-crossing the country in a last push for votes in key states.
Mr Obama has led in every national opinion poll since late September and is ahead with more than 300 electoral votes - far more than the 270 needed to capture the White House.
The campaigning ended on a cordial note, with John and Cindy McCain sending a message of condolence following the death of Mr Obama's grandmother. Mr Obama called it "incredibly gracious"
Meanwhile, Republican vice-presidential candidate Sarah Palin has been cleared by an investigator of abusing her power as governor of Alaska.
The report for the Alaska Personnel Board found that there was "no probable cause to believe that the governor, or any other state official" violated state ethics laws over the sacking of a state trooper who was once married to Ms Palin's sister.
High voter turnout expected
Some 130 million Americans are expected to vote, in a higher turnout than in any election since 1960, the BBC reports.
A record 27 million people had already cast absentee or early ballots as of Saturday night.
Under America's Electoral College system, states are apportioned votes based on their population, the biggest being California with 55 votes.
A candidate needs to gain 270 out of the 538 Electoral College votes to win the presidency.
Polls suggest the six closest state races on election day will be in Florida, Indiana, Missouri, North Carolina, Nevada and Ohio.
Mr McCain holds the lead in Indiana and North Carolina, but Mr Obama is ahead in the others, according to latest polls from Reuters/C-SPAN/Zogby suggest.
When Americans go to the polls, as well as choosing a new president and members of Congress, they will be casting votes on a wide range of ballot initiatives such as same-sex marriage, abortion and animal rights.