Republican vice presidential nominee governor Mike Pence spoke to supporters in Ohio about the importance of insuring the presidential election was fair, dismissing comments from the White House Press Secretary that he was not worried about voter fraud.

"I just heard today that President Obama's Press Secretary was asked if they were worried about vote rigging in the country, about whether it be an honest election, and his answer was quote, not at all, and neither is Mike Pence," he said. " Well, I got a news flash for you, the President's Press secretary doesn't speak for me."

"If you're here at a rally and you have not yet volunteered to participate in the electoral process in a respectful way, to be there at the precinct level, come election day you haven't yet done all that you can do," Pence said.

"They are not worried about it because they deny it is happening, I am not worried about it because I know the American people are not going to let it happen," he said.

Pence said the right to vote is at "the bedrock of our democracy" and told the crowd, "to respectfully participate to ensure the integrity of our democracy."

The country's top elected Republican, House of Representatives Speaker Paul Ryan, has tried to counter Donald Trump's message about election fraud. Spokeswoman AshLee Strong said on Monday that Ryan "is fully confident the states will carry out this election with integrity."

Ryan last week distanced himself from Trump, saying he was going to focus his election campaign efforts on trying to preserve the Republican majorities in Congress.

Since then, Trump has repeatedly sent out remarks via Twitter slamming Ryan.

In the traditionally closely fought state of Ohio, the top elections official, a Republican, said concerns about widespread voter fraud were simply not justified.

"I can reassure Donald Trump: I am in charge of elections in Ohio and they're not going to be rigged, I'll make sure of that," Ohio Secretary of State Jon Husted told CNN.

Numerous studies have shown that voter fraud in U.S. elections is very rare, according to the Brennan Center for Justice at New York University School of Law. In a report titled "The Truth About Voter Fraud," the center cited voter fraud incident rates between 0.00004 per cent and 0.0009 per cent.

The RealClearPolitics average of national opinion polls shows Clinton currently leading Trump by 5.5 percentage points, at 47.7 per cent support to his 42.5 per cent.