Clinton or Trump?

A nervous world turned its gaze to America's 200 million-strong electorate Tuesday as it chooses whether to send the first female president or a populist property tycoon to the White House.

Democratic frontrunner Hillary Clinton and Republican maverick Donald Trump campaigned into the wee hours of polling day as they fought to sell their starkly different visions for the future of the world's greatest power.

The 69-year old former first lady, senator and secretary of state -- backed by A-list musical stars and incumbent President Barack Obama -- urged the country to unite and vote for "a hopeful, inclusive, big-hearted America."

Trump meanwhile doubled down on his outreach to voters who feel left behind by globalization and social change, finishing with a flourish on his protectionist slogan: "America first."

"Just imagine what our country could accomplish if we started working together as one people, under one God, saluting one American flag," the 70-year-old billionaire reality television star told cheering supporters.

Some 40 million Americans have already cast ballots in states that allow early voting, and opinion polls suggest Clinton had a slight edge, as the tiny hamlet of Dixville Notch in New Hampshire opened Election Day balloting.

A polling average by tracker site RealClearPolitics gave Clinton a 3.3 percentage point national lead, but Trump is closer or even has the advantage in several of the swing states that he must conquer to pull off an upset.