Swing states hung in the balance over night as Donald Trump and Joe Biden both claimed they are on track for victory.
Major prizes including Florida appeared to go to Trump, a sign the polls were out ahead of the election.
But in a provocative message on social media, quickly deleted, Trump declared: "We are up BIG, but they are trying to STEAL the Election. We will never let them do it."
He hit social media while Biden tried to rally his supporters in a speech in Delaware.
Biden said: "Keep the faith, guys. We're gonna win this."
The former vice president hailed voters' patience, admitting the contest is tight.
"We knew this was going to go on but who knew it was going to go into maybe tomorrow morning, maybe even longer," he said.
"But look, we feel good about where we are. We really do. I am here to tell you tonight we believe we’re on track to win this election."
He later added: "I'm grateful to the poll workers, to our volunteers our canvassers, everyone who participated in this democratic process.
"And I'm grateful to all my supporters here in Delaware and all across the nation.
"Thank you, thank you, thank you."
Early results showed Trump won Ohia and Florida, big tests in the election.
The challenge for the presidency remained too close to call, but the Republican incumbent appeared to be performing better in the ballots than had been anticipated by polling leading up to election day.
The winner needs to win at least 270 electoral college votes - totals awarded for victory in each state, rather than individual votes by electors.
