Leesburg, United States: White House rivals Hillary Clinton and Donald Trump were due to duel Monday for a handful of must-win states in an end-game election frenzy capping a historically divisive campaign.

With less than 48 hours until voting day, it was unclear whether the Democrat could convert into electoral gain the announcement Sunday that the FBI had cleared her again of wrongdoing over her email use. 

Clinton's popularity had dipped after FBI Director James Comey dropped a campaign bombshell eight days earlier with a reopened inquiry into whether she exposed US secrets by using a private email server while serving as secretary of state.

Opinion polls had tightened as Trump began to recover ground lost while battling accusations of sexual assault, and the race looked headed for a photo finish.

The billionaire Republican contender, who has whipped up a populist grassroots movement among largely white male voters, landed in Sarasota, Florida early Monday and was met by an enthusiastic crowd.

After a stop there, the 70-year-old is due to fly to rallies in North Carolina, Pennsylvania, New Hampshire, before ending with a late event in Michigan.

In Virginia Sunday, he stoked supporters at a post-midnight rally with stock attacks on his rival, branding her the "most corrupt candidate ever to seek the office of the presidency."

- 'Brexit plus, plus, plus' -

Clinton, 69, had events planned through midnight Monday, with stops in Pennsylvania, Michigan, and North Carolina. The rallies will include appearances by a star-studded roster of supporters headlined by President Barack Obama and rock star Bruce Springsteen.

Clinton's Sunday rallies had a note of optimism mixed with warnings of the threat posed by Trump.

"I really want each and every one of us to think for a moment about how we would feel on November 9, if we were not successful," she said in Manchester, New Hampshire 

"When your kids and grandkids ask you what you did in 2016, when everything was on the line, I hope you'll be able to say you voted for a better, stronger, fairer America."

The world has looked on aghast as Trump's sensationalist reality television style became a driving force propelling him toward the most powerful political post in the world.

Global financial markets were rocked when the renewed FBI probe threatened to sink Clinton's chances, and were boosted by news of the FBI's closure of the affair. Asian and European exchanges jumped Monday morning, hours after the announcement.

But commentators said renewal of the email scandal, which dominated one of the last news cycles ahead of the election, had already damaged the Democratic former first lady's chance of succeeding Obama.

Clinton's lead dropped from 5.7 to 2.9 percentage points in the week since the scandal returned, according to influential data website FiveThirtyEight.com.

Trump is predicting a ballot upset on par with Britain's shock vote to quit the European Union, or what on Sunday he called: "Brexit plus, plus, plus." 

- 'Core values' tested -

"The rank and file special agents of the FBI won't let her get away with her terrible crimes," Trump told a rally in Michigan, a state Obama won comfortably in 2012.

For Trump and his supporters, Clinton symbolizes the corruption of the Washington elite.

"Right now she's being protected by a rigged system. It's a totally rigged system. I've been saying it for a long time," he declared, as his supporters chanted "Lock her up!"

Trump has repeatedly condemned Clinton's "criminal scheme" and argued that she's unfit to be president.

He has previously threatened to reject the result of Tuesday's vote if he loses, alleging that the race has been "rigged" by the media and the establishment elite.

Opinion polls tightened over the last week as Trump began to recover ground he lost after several women accused him of sexual assault, and the race looked headed for a photo finish.

Clinton made no direct reference to the FBI reprieve during her Sunday campaign stops.

Instead, she hammered her opponent over his sometimes ugly rhetoric and, implicitly, the alleged covert Russian interference that has poisoned the race.

"There are powerful forces inside and outside of America that do threaten to pull us apart," she said.

"We've arrived at a moment of reckoning in this election. Our core values as Americans are being tested."

If Clinton wins, she will seek to build on Obama's cautious but progressive legacy, including his controversial health insurance reforms.

Trump has vowed to tear up the reform along with free trade agreements, to rebuild a "depleted" US military, and review US alliances.

The latest polls give Clinton a narrow national lead of between three and five percentage points, but rolling averages point to a closer race, with Trump up in some swing states.

Silver has Clinton as a two-to-one favorite against Trump, but warned Sunday that her lead appears "less solid" than Obama's did before his re-election victory in 2012.