The result in the key US state of Ohio is too close to call this morning, with long lines at polling places and late legal manoeuvring adding to the tension.
Television networks were unable to project which camp would collect Ohio's 20 electoral votes, one of the biggest prizes of the US presidential election.
Long lines at polling places that stayed open more than two hours past the scheduled 7.30 p.m. closing time prompted assurances from election officials that those in line would get to vote.
The Democratic Party obtained an order in federal court requiring election officials in two central counties to let voters use paper punch card ballots as well as touch-screens to speed up the lines.
But the Ohio attorney general filed an appeal of the ruling, adding to the confusion.
One voter in Oberlin, in northwest Ohio, said the line she was in was three hours long. The Democratic Party complained of lines up to five hours long in some precincts.
The legal wrangling on election day included another federal judge's ruling that those who failed to receive absentee ballots must be allowed to cast provisional votes, and an appeals court ruling that allowed Republican and Democratic party observers into polling places to challenge suspect voters.
Despite concerns of widespread challenges to voters' status, few such challenges were lodged.