With four days to go until the election and time running out to sway undecided voters, President Barack Obama is stumping for Hillary Clinton on Thursday in the bitterly contested must-win state of Florida.

Republican Donald Trump will hold a rival rally in the city of Jacksonville hours before Obama's event, highlighting both candidates' laser focus on Florida with the race tightening in its final days.

Clinton painted a grim picture of life under a Trump presidency as she invited voters to envisage her defeat.

"Imagine it is January 20, 2017 and imagine that it is Donald Trump standing in front of the Capitol," she told a 15,000-strong crowd in Tempe, Arizona, triggering a chorus of boos for her Republican opponent.

"Imagine that he is taking the oath of office and then imagine that he is in the Oval Office making the decisions that affect your lives and your future," she said.

Clinton painted a picture of Trump as a president who demeans women, exacerbates racial divisions and is so thin-skinned and unpredictable that he could "start a real war instead of a Twitter war."

The note of caution was echoed by Obama, who warned voters that America's very future was at stake.

"The fate of the republic rests on your shoulders," he declared in North Carolina, one of a handful of swing states where the race will be decided.

"The fate of the world is teetering and you, North Carolina, are going to have to make sure that we push it in the right direction," Obama declared.

The 70-year-old Republican, by contrast, treated supporters in Florida to a now familiar tirade, predicting Clinton's downfall and vowing to "drain the swamp" of corruption in Washington.

Appearing at a triumphal rally in Pensacola, Trump also highlighted his need to stay on message and avoid the self-inflicted gaffes that have marred his White House run.

"Nice and cool. Right? Stay on point Donald, stay on point. No sidetracks Donald, nice and easy," he said.

Trump boasted that many opinion-makers and voters are now flocking to his standard. "We're only left with one person, crooked Hillary Clinton."

Such talk - partly supported by one poll on Tuesday showing him moving slightly ahead of Clinton - has delighted America's foes, made its allies queasy and spooked financial markets.