Here we explain the ‘mail-in’ voting that’s so critical in this election.
What is mail-in voting?
Most US states allow voters to apply for a postal ballot, known as a ‘mail-in’ or ‘absentee’ ballot.
Applications surged because of coronavirus as a way for voters to avoid queues, and more than 100million completed mail-in ballots.
What is Trump’s complaint? For months, he has been raising concerns about fraud. He currently seems to be focused on the deadlines by which votes should be cast
What are the rules?
Every state has its own system. Some require voters to show that they are unable to vote in person because, for example, they are in the military. Others allow any registered voter to apply.
There are also differences over the deadline. About 20 states allow ballots received after election day to be counted if they were postmarked by the day of the vote.
That includes Pennsylvania, one of the swing states.
Are they counted all at once?
No. Some states started counting on the morning of election day so they could post a quick result.
But others did not start until the morning after, once all the in-person votes had been tallied. That led to shifts in results, mainly eating into Trump leads.
What is Trump’s complaint?
For months, he has been raising concerns about fraud. He currently seems to be focused on the deadlines by which votes should be cast.
Mr Trump said: ‘We want all voting to stop’ and vowed to take action in the US Supreme Court.
He appeared to be confusing voting with counting. Joe Biden urged supporters to be patient until all votes are counted.
What could an action involve?
Unclear. But it could focus on postmarks, or their absence, on mail-in envelopes. It could also attempt to pick broader holes in the legitimacy of mail-in systems operated by individual states.
Have other presidential votes been disputed?
In the 2000 race between Republican George W Bush and Democrat Al Gore the result was disputed in Florida.
The issue was finally decided in the Supreme Court. The judges halted a recount, meaning Mr Bush won by 537 votes and became President.
Where would any legal challenge be decided?
Litigation would most likely be filed in individual states. But it could eventually reach the Supreme Court.
Just days before the election Mr Trump appointed a new judge, Amy Coney Barrett, as a Supreme Court justice, creating a 6-3 conservative majority that could favour him.
When could all this begin?
Expect Mr Trump’s team to move quickly. In fact, low-level litigation has already begun.
On Tuesday, two Pennsylvania Republicans filed a lawsuit claiming election officials in a Philadelphia suburb had unlawfully counted mail-in ballots before election day.
The lawsuit also claims that only some voters whose mail-in ballots contained errors had been contacted to correct them.
Where would any legal challenge be decided? Litigation would most likely be filed in individual states. But it could eventually reach the Supreme Court (pictured)
How long will it take?
In the Bush vs Gore case, the ruling came five weeks after election day. If the Trump campaign launches legal action in multiple states, it could take much longer.
Is mail-in voting as susceptible to fraud as Mr Trump claims?
Most states ask applicants to prove they are registered voters with information such as a social security number.
Signatures on the polling envelope may also be compared with records on file.
Mr Trump has voted by post in previous elections. He has voiced fears about the US Postal Service’s ability to handle mail-ins – and his Postmaster General Louis DeJoy, a major Republican donor, has been accused of deliberately running down the service and ripping out post boxes.
Which party benefits the most from postal voting?
Traditionally, postal votes were seen to favour Democrats who were more likely to be younger, on low incomes or from ethnic minorities.
They have also been more receptive to warnings about Covid and avoiding crowds.
However, a counter-argument is that the Republicans also benefit because their supporters are more likely to be older and perhaps infirm.