Donald Trump has claimed the result of November’s election may never be known, warning that “crazy Nancy Pelosi” could become president instead.
Mr Trump, who is behind Democrat rival Joe Biden in many polls, was speaking ahead of next week’s Republican National Convention.
He said: “You’re not going to be able to know the end of this election, in my opinion, for weeks, months, maybe never – maybe years but maybe never.
“There’s a theory that if you don’t have it by the end of the year, crazy Nancy Pelosi (Speaker of the House) would become president, you know that, right?
“No no, think of that, think of that – that mad theory too, you have heard that theory.
“Now I don’t know if it’s a theory or a fact but I said: ‘that’s not good’.”
Just hours earlier, former vice president Joe Biden vowed to end America’s “season of darkness” under Donald Trump as he accepted the Democratic presidential nomination.
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In a speech to end the Democratic Convention in Delaware, Mr Biden said the US president had “cloaked America in darkness for much too long”, accusing him of creating “too much anger, too much fear, too much division”.
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“Here and now I give you my word, if you entrust me with the presidency, I will draw on the best of us not the worst,” Mr Biden said.
Mr Biden also criticised his opponent’s response to the coronavirus pandemic – the US has the world’s highest recorded death toll from the virus.
But Mr Trump said on Friday that where Mr Biden saw “American darkness”, he sees “American greatness”.
He said: “Over the last week, the Democrats held the darkest and angriest and gloomiest convention in American history.
“They spent four straight days attacking America as racist, a horrible country that must be redeemed.”
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Mr Trump has portrayed himself as a defender of law and order but Mr Biden has described him as someone who tries to divide Americans.
Vice President Mike Pence, who is also running to be reelected on the Republican ticket, said next week’s party convention would focus on what Mr Trump had accomplished, including on the economy and coronavirus.
He promised supporters that a “great line-up of leaders” would join the convention to “talk about what this president has done”.
Among the speakers are Mr Trump, Mr Pence and Mr Trump’s wife, first lady Melania.