US President Donald Trump has denied reports he plans to declare an early election victory, but signalled legal challenges to prevent ballots from being counted after election day.

The Axios website earlier reported that Mr Trump and his campaign aides were planning to declare victory in Wednesday’s US presidential election before the millions of mail-in votes were counted.

“Trump has privately talked through this scenario in some detail in the last few weeks, describing plans to walk up to a podium on election night and declare he has won,” Jonathan Swan, national political correspondent for political site Axios reports.

More than 91 million Americans have already posted their votes, with some states receiving more early votes than their state’s entire 2016 tally.

Mr Trump has long maintained unsubstantiated claims that postal ballots are not secure and that Democrats would try to steal the election.

“I think it’s a terrible thing when ballots can be collected after an election,” the President said in Charlotte, North Carolina, on Monday Australian time.

“I think it’s a terrible thing when people or states are allowed to tabulate ballots for a long period of time after the election is over, because it can only lead to one thing,” Mr Trump said, criticising decisions by the Supreme Court to allow ballots to be received after election day in several battleground states. 

“As soon as the election is over – we’re going in with our lawyers,” he said, mentioning the state of Pennsylvania specifically.

Democratic election hopeful Joe Biden earlier said “the President is not going to steal this election.”

Postal votes have been counted after election day in the US for decades, with many states candidates not decided for days or weeks after polling day.

An election worker carries mail-in ballots in US Postal Service containers to be processed by election workers at the Salt Lake County election office in Salt Lake City, Utah on October 29. Photo: Getty

Most states expect to be able to complete counting mail-in votes in just a few days after election day.

Mr Trump would need to be heavily leading, or have already won, the states of Ohio, Florida, North Carolina, Texas, Iowa, Arizona and Georgia to be able to attempt to claim victory.

“This is nothing but people trying to create doubt about a Trump victory. When he wins, he’s going to say so.” Trump campaign communications director Tim Murtaugh was quoted as saying by Axios. 

Opinion polls show Mr Trump trailing former vice-president Biden nationally, but with a closer contest in the most competitive states that will decide the election. Voters say the coronavirus is their top concern.

In a flurry of legal motions, his campaign has sought to restrict absentee balloting.

Former President Obama joined Mr Biden in the final days of the campaign. Photo: Getty

“I don’t care how hard Donald Trump tries. There’s nothing – let me say that again – there’s nothing that he can do to stop the people of this nation from voting in overwhelming numbers and taking back this democracy,” Mr Biden said on Saturday at a rally in Flint, Michigan, where he was joined by former president Barack Obama for their first 2020 campaign event together.

Mr Trump held four rallies on Saturday in the battleground state of Pennsylvania, where the campaigns are seeking to win over undecided voters in areas such as the suburbs of Philadelphia and the “Rust Belt” west of the state.

“If we win Pennsylvania, it’s over,” Mr Trump told a large rally in Reading before moving to another big gathering in Butler.

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Officials in several states, including Pennsylvania and Wisconsin, say it could take several days to count all the mail ballots, possibly leading to days of uncertainty if the outcome hinges on those states.

A federal judge in Texas has scheduled an emergency hearing for Monday on whether Houston officials unlawfully allowed drive-through voting and should toss more than 100,000 votes in Democratic-leaning Harris County.

In Iowa, a new poll shows Mr Trump has taken over the lead there.

The Des Moines Register/Mediacom Iowa poll shows Mr Trump now leads Mr Biden by seven percentage points.

-with AAP