US President Donald Trump has declined to commit to a peaceful transfer of power if he loses the 3 November election to Democratic rival Joe Biden.
Mr Trump has also said he expects the election battle to end up before the Supreme Court.
"We're going to have to see what happens," President Trump, a Republican, told reporters at the White House when asked whether he would commit to transferring power.
The president, who trails Mr Biden in national opinion polls, has repeatedly cast doubt on the legitimacy of the election, asserting without evidence that mail-in voting would lead to fraud and a "rigged" outcome.
"The ballots are a disaster," Mr Trump said.
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Democrats have encouraged voting by mail as a way to cast ballots safely during the coronavirus pandemic.
Millions of Americans, including much of the military, have cast absentee ballots by mail for years without problems.
In 2016, President Trump also raised questions about whether he would accept the results of the election. He went on to win the presidency.
Mr Biden, speaking to reporters in Delaware, said Mr Trump's comments on the transition of power were "irrational".
His campaign said it was prepared for any "shenanigans" from President Trump, and reiterated comments from July that "the United States government is perfectly capable of escorting trespassers out of the White House".
Republican Senator Mitt Romney said on Twitter: "Fundamental to democracy is the peaceful transition of power; without that, there is Belarus.
"Any suggestion that a president might not respect this Constitutional guarantee is both unthinkable and unacceptable."
Fundamental to democracy is the peaceful transition of power; without that, there is Belarus. Any suggestion that a president might not respect this Constitutional guarantee is both unthinkable and unacceptable.— Mitt Romney (@MittRomney) September 24, 2020
President Trump, who is moving quickly to nominate a successor to liberal Justice Ruth Bader Ginsburg, who died on Friday, said earlier in the day he thinks the election "will end up in the Supreme Court and I think it's very important that we have nine justices".
He plans to announce his nominee on Saturday.
A Senate confirmation vote before the election would seal a 6-3 conservative majority on the court, potentially spelling trouble for Democrats should it be called on to decide any legal dispute over the results of the election.
"This scam that the Democrats are pulling, it's a scam, the scam will be before the United States Supreme Court, and I think having a 4-4 situation is not a good situation," Mr Trump said.
Only one US presidential election, the 2000 contest between Republican George W Bush and Democrat Al Gore, has had its outcome determined by the Supreme Court.
Earlier, in Charlotte, North Carolina, where he was campaigning, Mr Biden said Republicans were "violating the essence" of what the country's founders had intended in quickly seeking to fill the Supreme Court seat.
"This is an abuse of power of what they're doing, and I think we should focus on what this is going to mean for healthcare," Mr Biden said, suggesting such a conservative majority could roll back abortion rights and the Obamacare healthcare law.
President Trump has already appointed two conservatives to lifetime posts on the court, Neil Gorsuch in 2017 and Brett Kavanaugh in 2018.
Supreme Court appointments require Senate confirmation, and Republicans hold 53 of the chamber's 100 seats.
Senate Majority Leader Mitch McConnell has said he intends to act on any nomination Mr Trump makes.
Democrats accuse Mr McConnell of hypocrisy, pointing out he refused to have the Senate even consider Democratic President Barack Obama's nominee to fill a vacant Supreme Court seat in 2016 because it was an election year.
He has said there would be an"orderly" transition following the presidential election.
"The winner of the November 3rd election will be inaugurated on January 20th. There will be an orderly transition just as there has been every four years since 1792", Mr McConnell tweeted.
The winner of the November 3rd election will be inaugurated on January 20th. There will be an orderly transition just as there has been every four years since 1792.— Leader McConnell (@senatemajldr) September 24, 2020
US election: Things 'closer than they look'
Kim Darroch, former UK Ambassador to the US, said he suspects things are "closer than they look" in the run up to the US general election and there is "still a lot to play for".
Mr Darroch said the debates have yet to come and these can play a critical role in people's decisions.
He told RTÉ’s Morning Ireland that random events will also affect how people vote and noted that the polls show people still have more confidence in the Republican economic policies than the Democratic ones.
Mr Darroch said that you can be sure that "the Trump vote will come out", adding that the Democrat vote did not turn out sufficiently for Hillary Clinton in 2016 and Joe Biden must mobilise this vote in November.
If Joe Biden wins the election, he said, then it is likely that trade deals would be centred on a EU/US free trade deal, which would be much bigger than a US deal with the UK.
Mr Darroch said they also could be based on the US joining the Transpacific Partnership.
Should Donald Trump win the election, he said a UK/US free trade deal would be prioritised but that the Democrats have the power to block such a deal, adding that Democrat concerns about the Good Friday Agreement would override economic concerns.
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