Hillary Clinton, who lost to US President Donald Trump in 2016, said Tuesday that 2020's Democratic challenger Joe Biden should refuse to concede until all votes are counted in a tight and possibly drawn-out contest.

Clinton said that possible delays in tallying ballots - with mail-in voting set to reach unprecedented levels - mean Biden must hold back from accepting defeat initially.

"Joe Biden should not concede under any circumstances because I think this is going to drag out and, eventually, I do believe he will win if we don't give an inch and if we are as focused and relentless as the other side," Clinton said in an interview with Showtime's "The Circus."

Clinton, who won almost 3 million more votes than Trump overall in 2016 but lost to him in the state-by-state electoral college count, suggested that the Republicans are hoping to get the result they want through in-person voting on the day - and not necessarily wait for any delayed mail-in ballots to be counted.

Her comments follow warnings from both sides that the result of the election may not be known on the evening of November 3.

Republicans "have a couple of scenarios that they are looking toward. One is messing up absentee balloting," she said.

"They believe that helps them so that they then get maybe a narrow advantage in the electoral college on election day," Clinton said.

"We have to have our own teams of people to counter the force of intimidation that the Republicans and Trump are going to put outside polling places," she said.

Americans are expected to vote by mail in massive numbers due to the coronavirus pandemic, but Trump has opposed more funding for the US Postal Service, acknowledging the money would be used to help process ballots.

Trump says, without providing evidence, that mail-in voting will lead to mass fraud and a rigged election. The practice is already common in parts of the US, without significant fraud, and Trump himself votes by mail.

Democrats accuse Trump of deliberately trying to sabotage the post office's capacity to deal with the expected big increase in volume.

Trump has repeatedly refused to confirm that he will accept the result of the election. Opinion polls show him currently behind Biden both in the popular and electoral college contests.

AFP