When polls closed in an Arizona Senate race in November 2018, Republican Martha McSally had a narrow lead over Democrat Kyrsten Sinema. But that edge evaporated in the days that followed. The reason: The time it took to tally mail-in ballots.
The Republican president was furious: “Electoral corruption - call for a new election?” Donald Trump posted on Twitter.
There was no new election, and McSally conceded to Sinema six days after Election Day.
But now some fear that scenario could play out in the U.S. general election, with Trump ahead in early returns and Democrat Joe Biden emerging as the winner in the days that follow.
According to Reuters/Ipsos polling, nearly half of Democrats say they plan to vote by mail, while only one quarter of Republicans plan to do so.
Demand is driven in no small part by fear of infection from the coronavirus in public.
People close to Biden's campaign say they are bracing for Trump to complain the contest is being stolen as the lead shifts.
Even some Republicans worry that Trump might exploit the uncertainty to cast doubt on the results if he ends up losing.
In more than 30 states, including such battleground as Arizona, Florida and North Carolina that traditionally decide elections, officials can begin processing or even counting ballots before Election Day.
But three Rust Belt states considered among the most competitive - Michigan, Pennsylvania and Wisconsin - prohibit officials from processing, let alone counting, ballots before Election Day.
Trump has refused to say whether he will respect the outcome if he loses.
And he has repeatedly made unfounded claims that mail voting leads to fraud, even though multiple studies show voter fraud in the United States is extremely rare.