PITTSBURGH -  Hillary Clinton and Donald Trump battled for votes on a frenzied final day of campaigning Monday, telling Americans the country's fate rides on who they choose as the next US president.

Clinton, the front-running Democrat, aimed to nail down her narrow lead with stops in three battleground states, as President Barack Obama covered for her elsewhere before they join up at a star-studded grand finale in Philadelphia.

"The choice in this election could not be clearer," Clinton said at a rally in Pittsburgh, Pennsylvania. "It really is between division or unity. Between strong and steady leadership or a loose cannon."

Trump, the billionaire Republican nominee, set out from must-win Florida on a five state swing, the culmination of a dramatic run for the presidency as a right-wing nationalist vowing radical change in America's relationship with the world.

"I want the entire corrupt Washington establishment to hear the words we are all about to say: when we win tomorrow, we are going to drain the swamp," he told cheering supporters in Sarasota, Florida.

Chants of "Drain the swamp! Drain the swamp" rose from the crowd.

Despite his outward confidence, the 70-year-old mogul needs to break through a wall of Democratic support in industrial northern states like Michigan to win.

Trump, Clinton and Obama all focused precious final efforts campaigning there and other key states.

As she boarded her campaign plane in White Plains, New York for the day's first rally in Pennsylvania, Clinton admitted that bringing the country together again after one of the bitterest US elections ever will require "some work."

"I really do want to be the president for everybody - people who vote for me, people who vote against me," she told reporters.

In a video message set to air during two prime-time television shows reaching millions of viewers, Clinton warns, "Our core values are being tested in this election."

"Is America dark and divisive, or hopeful and inclusive?" she asks as a piano trills in the background.

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

US media predicted substantial to big wins for the 69-year-old Democrat when electoral votes are counted. Influential election forecaster FiveThirtyEight gave her a two to one chance of winning the 270 votes needed to claim the White House.

Under the cloud of an FBI investigation, Clinton got good news Sunday when James Comey, the agency's director, cleared her again of criminal wrongdoing in her use of a private email server while serving as secretary of state.

A Clinton spokeswoman said the candidate was “glad this matter is resolved”. “We’re glad to see that (FBI) has found, as we were confident that he would, that he has confirmed the conclusions he reached in July,” director of communications Jennifer Palmieri said in a statement.

The Democratic nominee’s opponent, Republican Donald Trump, reacted with anger at the news, and cast doubt on whether the FBI had even carried out its work. “You can’t review 650,000 emails in eight days,” Trump told a campaign rally in Sterling Heights, Michigan, on Sunday evening.

On 28 October, only 11 days before the presidential election, FBI Director James Comey sent congressional leaders a letter informing them that agents had discovered emails “that appear pertinent” to a prior investigation, into Clinton’s use of a private server while she was secretary of state. It was later reported that as many as 650,000 such emails were in question.

The move, so close to an election, proved tremendously controversial. In July, Comey had announced that Clinton and her aides were “extremely careless” but that “no reasonable prosecutor” would bring a case against them.

“Since my letter, the FBI investigative team has been working around the clock to process and review a large volume of emails from a device obtained in connection with an unrelated criminal investigation,” Comey wrote to Congress on Sunday.

“During that process, we reviewed all of the communications that were to or from Hillary Clinton while she was secretary of state.”“Based on our review, we have not changed our conclusions that we expressed in July with respect to Secretary Clinton,” he concluded. “I am grateful to the professionals at the FBI for doing an extraordinary amount of high-quality work in a short period of time.”

Clinton campaign spokesman Brian Fallon said their campaign had been validated in its belief that nothing would change.

“We were always confident nothing would cause the July decision to be revisited. Now Director Comey has confirmed it,” he tweeted

The Trump campaign was unbowed and swift to fire back, predictably slamming Comey and the bureau over the outcome and refusing to resile from its repeated allegations of Clinton’s criminality.

In Michigan, Trump called Clinton “the most corrupt person ever to seek the office of the presidency of the United States”.

“Well, you have to understand it’s a rigged system and she’s protected,” Trump said at another rally in Minnesota.

Trump’s spokesman Jason Miller told CNN soon after the decision on Sunday evening (US time): “We thought that Director Comey and the FBI were wrong when they made their initial recommendation in July, and we think that they’re wrong now.”

Trump's combative campaign manager Kellyanne Conway took the same line, alleging Comey and the FBI had mishandled the investigation.

US stocks jumped and the US dollar and Mexican peso soared after the FBI’s announcement.

The latest FBI review of the emails had rattled financial markets and so the latest news triggered a relief rally.

The news boosted US S&P 500 Index futures 1.2 per cent, a gain that is likely to snap the nine-day losing streak in the US stock index – its longest in more than 35 years. MSCI’s broadest index of Asia-Pacific shares outside Japan rose 0.2 per cent as Australian shares gained 0.7 per cent.

Japan’s Nikkei looks set to gain 1.5 per cent based on Chicago-traded futures.

In the currency market, the dollar rose as much as 1.4 per cent against the yen and last stood at 103.96 yen, up 0.8 per cent from late US levels while the euro dropped 0.5 per cent to $US1.1089.

The biggest winner was the Mexican peso, which has acted as something of a bellwether of sentiment as Trump’s proposed policies are considered to be deeply negative for the country. The peso rose 2.2 per cent to 18.61 to the dollar, hitting its highest level since Oct 26.