Donald Trump will lay out plans Saturday for the first 100 days of his presidency, in what his campaign is calling his "closing arguments" in one of the most bitter election campaigns in US history.

The 2016 election cycle pitting the Republican nominee against former secretary of state Hillary Clinton has turned increasingly toxic, with Trump fueling wild conspiracy theories about vote "rigging" and Clinton warning that the provocative billionaire was straying into authoritarianism.

Clinton excoriated Trump as a threat to American democracy Friday for not pledging to honor results of the upcoming presidential election, as the rivals battled for supremacy in battleground states.

"We know the difference between leadership and dictatorship, and the peaceful transition of power is one of the things that sets us apart," Clinton told a rally in Cleveland, Ohio, one of the key swing states up for grabs on November 8.

"Donald Trump refused to say that he'd respect the results of this election. By doing that, he's threatening our democracy."

Her comments marked a stern rebuke to Trump's bombshell suggestion during their third and final presidential debate that he may not recognize the election result -- a surprising rejection of political norms.

Trump, 70, then told a rally crowd that he could launch a legal challenge if Clinton prevails.

His remarks follow weeks of Trump warning about the likelihood of a "rigged" election including massive voter fraud, despite members of his own party disavowing the comments and Trump drawing condemnation from President Barack Obama.

Despite isolated allegations of voter fraud, controversy over the tight 2000 vote and rampant gerrymandering, US elections have been regarded as free and fair.

Invigorated by both her commanding poll numbers and Trump's eyebrow-raising declarations, the candidate vying to become America's first female president was in Ohio aiming to block Trump's efforts to claim the blue-collar heartland state.

Trump, well aware that no Republican has ever won the White House without winning Ohio, campaigned in the Buckeye State Thursday. He is due to head back to the state on Saturday, with running mate Mike Pence.

On Friday, the Manhattan real estate mogul hosted rallies in the battlegrounds of North Carolina and Pennsylvania.

"Eighteen days. You're going to look back at this election and say this is by far the most important vote you've ever cast for anyone at any time," Trump told a crowd in Fletcher, North Carolina.

- 'Win, lose or draw' -
On Saturday, he will make a key speech in Gettysburg, Pennsylvania, site of Abraham Lincoln's Gettysburg Address, one of the most famous speeches in American history, delivered during the Civil War in an effort to help unite the country.

"The Donald Trump campaign is a movement unlike anything we've seen in our country's history. Tomorrow's speech will set the tone for the closing arguments of this election," Trump's national policy director Stephen Miller said in a statement.

"Mr Trump is the change agent our country needs and he will speak to every American tomorrow about his positive vision to restore our economy, give government back to the people and outline the immediate steps he will take in the first 100 days to Make America Great Again."

Trump earlier said he would give the campaign everything he had, "right up until the actual vote."

"Win, lose or draw... I will be happy with myself," he added.

Clinton is narrowly leading in polling in North Carolina, a state Obama won in 2008 but lost to Republican Mitt Romney in 2012.

- Battleground focus -
Trump is trailing badly in the polls, and his debate threat opened him up to a stinging attack from Obama at a Miami rally.

"When you try to sow the seeds of doubt in people's minds about the legitimacy of our election, that undermines our democracy," Obama said Thursday.

"When you suggest rigging or fraud without a shred of evidence... That is not a joking matter."

Clinton holds leads in several battleground states, ranging from razor-thin, such as in North Carolina, to moderate in Florida and Pennsylvania and commanding in Virginia.

She is even narrowly ahead in Arizona, the traditionally Republican-leaning state where First Lady Michelle Obama -- who galvanized voters with a searing attack on Trump last week -- campaigned for Clinton Thursday.

If Trump loses Florida, Pennsylvania and North Carolina, Clinton is all but assured of victory, experts have said.

In Johnstown, Pennsylvania, Trump supporters streamed into a convention center to hear him speak in their depressed former steel town, where most mill jobs have evaporated.

Trump promised that he would bring many of them back.

"We don't make things anymore," he told the cheering crowd. "When I'm president, we're going to start making things again in America."