Donald Trump has a long history of claiming that US elections are being rigged, particularly when the results are not to his liking.
During the 2012 presidential campaign, when Trump championed the notorious "birther" theory that Barack Obama was not American, the property developer claims voting machines wiped ballots for Republican candidate Mitt Romney.
Just after the results were announced, Trump tweets -- wrongly -- that Obama "lost the popular vote by a lot and won the election. We should have a revolution in this country!
"We should march on Washington and stop this travesty," he adds.
During the Republican primaries in 2016, Trump accuses his rival Ted Cruz of cheating when he beat him convincingly in Iowa and Wisconsin.
"Ted Cruz didn't win Iowa, he stole it," Trump tweets. "That is why all of the polls were so wrong and why he got far more votes than anticipated. Bad!"
After he wins the Republican nomination, Trump ramps up his rhetoric, attacking the "corrupt" voting system as he fell behind in opinion polls to the Democrat Hillary Clinton, who he dubbed "Crooked Hillary".
On August 1 he twice warned that the election "is going to be rigged", telling supporters "we'd better be careful... or it's going to be taken away from us".
"People that have died 10 years ago are still voting," he later claims, saying "fraud is very, very common" and that illegal immigrants are also voting.
The candidate also spreads a conspiracy theory claiming that 2.5 million Clinton supporters have been given two votes "so they can vote twice".
During a Fox News presidential debate less than three weeks before the November 8 poll in 2016, Trump dramatically breaks with US electoral tradition by saying he may not accept defeat if Clinton wins.
"I will tell you at the time," he says. "I'll keep you in suspense."
Even after he wins the presidency with a majority of the electoral college, Trump claims wrongly that he also won the popular vote, saying Clinton got millions of illegal ballots.
"Serious voter fraud in Virginia, New Hampshire and California," he tweets.
"So why isn't the media reporting on this? Serious bias -- big problem!"
In the final months of the 2020 campaign, Trump repeatedly refuses to commit to a peaceful transfer of power if he loses, saying Democrats were using postal votes to steal the election.
Postal voting -- which often tilts Democrat -- was "a whole big scam", he claims.
He tweets that this "is the most INACCURATE & FRAUDULENT Election in history."
Hours after counting started, Trump tweets that "We are up BIG, but they are trying to STEAL the Election" and later says he will go to the Supreme Court to dispute the count.
Experts and US government officials have repeatedly said polling fraud is rare.