Both candidates are preparing for a legal battle in the potentially decisive swing state of Ohio amid fears that the result there may not be finalised for 10 days.
The state tends to have a high number of provisional ballots cast, and those are not counted until November 17. These ballots are cast by those who have applied for absentee ballots but then turn up to vote in person; those who have moved recently and not updated their address; those who do not provide proof of identity; and those whose signatures do not match that on registration forms.
In the past, Ohio has been second only to California in the number of provisional ballots cast. In 2008, they made up 207,000 out of a total of 5.7 million votes in the state. Some estimates put the number expected this time as high as 350,000.
Ohio, which has 18 electoral college votes, was expected to be a tight race. Mr Obama won the state by fewer than 300,000 votes four years ago.
Meanwhile, the US Embassy in Dublin hosted an Election Night Special for around 1,000 VIPs in the Guinness Storehouse last night.
Government ministers, TDs and leading figures from business, media and the arts gathered on the second floor in St James's Gate to watch massive screens relaying CNN and Fox News coverage of the race for the White House.
Although Ambassador Dan Rooney was absent, casting his vote in his home city of Pittsburgh, the 80-year-old managed to address the special guests of the Embassy via a live link from the US. Taking his place as host for the night was his Deputy Chief of Mission, John Hennessey-Niland.
Kicking off at 9pm, the event closed at 1am just as the first exit polls were giving an indication of who the next President of the United States would be. (© Daily Telegraph, London)