'Corrupt elite'
No results or exit polls will be available before polling stations begin to close on the US East Coast from 7:00 pm (0000 GMT Wednesday), and it may be three or more hours after that before the direction of the race becomes clear.
And even then, questions remain. Trump has repeatedly warned that a "corrupt Washington and media elite" is seeking to rig the race and he said last month that he may not concede defeat if he thinks voting is unfair.
He has also threatened to lodge lawsuits against up to a dozen women who have come forward during the race to accuse him of sexual assault or inappropriate behavior.
Clinton has pushed a more optimistic vision, despite a wobble in the final weeks of her campaign when the FBI reopened an investigation into whether she had put US secrets at risk by using a private email server -- only to close it again on Sunday.
In a radio interview on the last night of the race, she said the matter was behind her, and she courted voters at her final rallies in Philadelphia with Obama and rocker Bruce Springsteen, and in North Carolina with pop diva Lady Gaga.
"Tomorrow, we face the test of our time," she declared in front of 40,000 people in Philadelphia, a record for her in a campaign where despite her opinion poll lead she has struggled to match her Republican opponent's passionate and raucous crowds.
"There is a clear choice in this election. A choice between division or unity, an economy that works for everyone, or only for those at the top; between strong, steady leadership, or a loose cannon who could put everything at risk."
At the same time, Trump, who hijacked his conservative party and turned it into a vehicle for populist bombast, concluded a last-gasp tour of swing states by painting his rival as doomed to defeat and the corrupt creature of a discredited elite.