GRAND RAPIDS, United States - Hillary Clinton and Donald Trump have given US voters a stark choice as their brutal White House battle neared its end -- between her vision of unity and his promise to take back power from Washington’s corrupt elites.

On the final day of a presidential campaign that has left a bemused world looking on in concern for its greatest power, Trump on Monday doubled down on his unabashed appeal to popular resentment.

The Democrat Clinton has had sharp words for her rival, a 70-year-old property tycoon who seized the conservative Republican Party and turned it into a vehicle for his populist bombast, but she ended the race pleading for unity.

Some 40 million Americans have already cast ballots in states that allow early voting, and tens of millions more will turn out on Tuesday for what is shaping up to be a historic clash.

"It’s time to fight for America. I’m not a politician, I can say proudly. My only special interest is you," Trump told a raucous crowd, vowing to end free trade, control immigration and "bring back" jobs.

"But the years of betrayal will end -- they will end and they’re going to end quickly. They’re going to end as of the day we take office, which is going to be very, very soon."

Clinton -- the 69-year-old former first lady, senator and secretary of state -- has had a narrow but consistent lead in opinion polls as she strives to become America’s first woman president.

But the campaign has been bruising -- she has been dogged by allegations that she put US secrets at risk on her private email server while at the State Department and the race remains perilously close in a handful of key states.

Support for Trump dropped after footage emerged of him bragging about sexual assault and at least a dozen women came forward to accuse him, but polls have tightened as the big day approaches.

"Tomorrow, we face the test of our time," Clinton warned. "Will we be coming together as a nation, or splitting further apart? Will we set goals that all of us can help meet, or will we turn on each other."

"After tomorrow, the work will begin, and one of the highest priorities that I feel an obligation to address is how we bring the country together," the Democrat added, at a rally in Michigan.

Last-day blitz

As Election Day approached -- to the relief of the many voters left cold by the long drawn-out slog through the political mud -- both candidates maintained break-neck schedules.

Clinton made stops in three battleground states, and deployed President Barack Obama as cheerleader-in-chief in another, before they were to join up at a star-studded grand finale in Philadelphia.

"The world is watching us right now. This is one of those moments. Don’t let it slip away," Obama said in New Hampshire.

Trump sneered at Clinton’s choice of pop stars -- Bruce Springsteen and Lady Gaga -- to attract the crowds, and stuck to his stump speech, each new version slightly more aggressive than the last.

"This election will decide whether we are ruled by a corrupt political class, you’re seeing what’s happening. Everybody is watching. Or whether we are ruled by the people," he declared.

"We’re going to be ruled by the people, folks. That is going to be the choice. Our failed political establishment has delivered nothing but poverty, nothing but problems, nothing but losses.

"It’s time to reject the media and political elite that has bled our country dry," he declared, pointing to the press pen at his event, and whipping up cries of "Drain the swamp!" from his fervent supporters.

Market rally

Trump’s vow to rip up America’s free trade deals, build a wall on the Mexican border and renegotiate US treaty alliances has spooked world markets seeking stability after the recent global slowdown.

Last week, US stocks as measured by the S&P 500 index fell for nine straight days for the first time since 1980, only to recover a little when the FBI confirmed Clinton would not face prosecution over her emails.

Global stock markets surged on Monday, with Wall Street gaining more than two percent, as hopes for a Clinton victory rose after FBI director James Comey’s weekend announcement that she was in the clear.

The dollar also gained, and analyst Patrick O’Hare of briefing.com said: "You can certainly take for granted from today’s rally that the market seems to like the idea of Mrs. Clinton being elected president."

On the eve of the vote, Clinton held a widening but still close 3.2 percentage point lead over Trump in a four-way race including two fringe candidates, according to a RealClearPolitics average of national polls.

But a win in votes casts will mean nothing if she does not win the electoral college, fending off a last-ditch Trump bid to secure Florida and poach North Carolina, Michigan, Ohio, Nevada or New Hampshire.

On the eve of voting, respected data journalist Nate Silver’s site FiveThirtyEight.com gave the Democrat a 69 per cent chance of victory, conservative odds compared to newer rivals. - AFP