Donald Trump has claimed foreign countries will "print millions of ballot papers" to rig November’s presidential election.
In a bizarre all-caps tweet on Monday, the president said "it will be the scandal of our times".
The post said: "RIGGED 2020 ELECTION: MILLIONS OF MAIL-IN BALLOTS WILL BE PRINTED BY FOREIGN COUNTRIES, AND OTHERS. IT WILL BE THE SCANDAL OF OUR TIMES!"
One Twitter user replied: "Trump knows he’s gonna lose big time, so he’s already creating the excuse why."
Another said: "Trump's laying the groundwork for an election challenge."
A third wrote: "Anyone who knows how vote-by-mail works can explain just how impossible this would be."
It comes a day after Mr Trump's first rally since the coronavirus lockdown, which gained a lower turnout than expected.
Plans for an "overflow rally" for people who couldn't get tickets to the main arena in Tulsa, Oklahoma were abandoned, while photographs showed many empty seats inside the 19,000-seat capacity arena.
The president focused on differences between him and Joe Biden - his Democratic rival in the 2020 election.
He told supporters at the rally: "The choice in 2020 is very simple. Do you want to bow before the left-wing mob, or do you want to stand up tall and proud as Americans?"
Meanwhile, John Bolton, the president's former security advisor, said he will not vote for Mr Trump in the upcoming election, saying: “I don’t think he’s fit for office.”
In an interview with ABC News, Mr Bolton said he hopes history will remember Mr Trump as a "one-term president who didn’t plunge the country irretrievably into a downward spiral".
Mr Trump revealed on Sunday that he had ordered officials to slow down coronavirus testing rates to avoid the appearance of alarming numbers of cases as the virus spread through the country.
He told the campaign rally that testing was a "double-edged sword" because it revealed the extent of the virus.
He said: "Here is the bad part: When you do testing to that extent, you are going to find more people, you will find more cases. So I said 'slow the testing down'. They test and they test."
An official for the Trump campaign later said he was "clearly joking".
