Democratic presidential candidate Hillary Clinton began the night with a lead of more than three percentage points, but high spirits were quickly eroded as Republican rival Donald Trump proved hard to shake in the swing state of Florida.
Although Mrs Clinton has clinched a hard fought win in Virginia, which was supposed to be a Democratic stronghold, Mr Trump took the swing state of Ohio.
The race is also tight in North Carolina, as well as Wisconsin, Michigan, Virginia and New Hampshire, previously thought to be Democratic strongholds.
The evening began upbeat at the Jacob Javits Center in New York, where Mrs Clinton will give her victory or concession speech.
Supporters were gathered under a literal glass ceiling meant to symbolise the invisible barrier to women's achievements that Mrs Clinton may surmount.
"Thinking about my daughter right now," her running mate Tim Kaine wrote on Twitter. "No little girl will ever again have to wonder whether she, too, can be president."
But just half an hour later, Mrs Clinton's campaign posted a more measured statement that acknowledged the possibility of being edged out in the polls: "Whatever happens tonight, thank you for everything."
CNN is describing the scene at the Clinton camp as showing signs of a "dramatic mood shift", with the deflation in mood - and election prospects - attributed to faulty modelling.
According to Mother Jones reporter David Corn, a Democratic senator has called the nail-biting finish "crazy s***".
Fox News reports that several Clinton aides say they are "expecting a long night" but that Mrs Clinton is no stranger to having "had to fight for everything, her entire life".
Broadway sensation and creator of the hit musical Hamilton, Lin-Manuel Miranda, who has endorsed Mrs Clinton, was on Twitter and Facebook comparing the prevailing mood with his experiences in previous elections.
"I am on a Google chat with my best college friends, with whom I weathered '00, '04, '08, '12," he wrote. "Their faces are making this so much better."
Mr Trump is currently up in the popular vote. Eyes around the world remain fixed on the electoral map as reporting moves westwards.
