The 2020 US presidential election is set to be one of the most bitter in history, with threats of litigation compounded by a coronavirus pandemic that could deter people from voting in person.
Donald Trump has refused to say whether he will accept the result of the election this year, has hinted that the outcome may have to be settled by the Supreme Court, and has repeatedly raised unfounded allegations that increased use of mail-in votes could lead to fraud.
At the same time, a record number of Americans will be using mail-in votes, with many preferring not to risk the chance of being infected with COVID-19 in a country which has seen more than 210,000 deaths.
With four weeks to go, more than four million Americans have already voted – more than 50 times greater than the 75,000 who had voted at this stage in 2016.
But can the US Postal Service cope with this huge demand and can Mr Trump challenge the outcome of the election?
Early voting numbers show Americans are rushing to cast their ballots ahead of the 3 November election at an unprecedented pace.
More from Donald Trump
US Election 2020: Tears from the Trump faithful – but bitter divides remain in Pennsylvania
Amy Coney Barrett: Trump’s Supreme Court pick dodges key questions at confirmation hearing
US Election 2020: How the US economy has performed under Trump in five charts
US election: What is Blexit, who are the Blexiteers and what do they want?
US election 2020: Joe Biden heads to Ohio in bid to win over voters in Republican stronghold state
US Election 2020: Trump flies in to woo Florida – but the grey vote might be turning against him
Please use Chrome browser for a more accessible video player
At the end of July, the US Postal Service wrote to 46 states to warn them it could not guarantee that all mail-in ballots will arrive in time to be counted.
That means those ballots could be rendered invalid and risks disenfranchising a significant chunk of the electorate.
This week, the US Postal Service asked for a clarification after four US judges issued preliminary injunctions barring it from making service reductions ahead of the election.
It warned the 27 September ruling “would undermine the Postal Service’s ability to timely deliver the mail before upcoming election”.
Each US state sets its own deadline for when mail-in votes must arrive and still be counted, but in the key battleground states many of these decisions are being challenged in court.
Court battles have spread to every competitive state in the US, with election-related litigation focused on issues including witness signatures, US mail postmarks and the use of drop boxes for ballots.
More than 200 election-related cases are currently pending in the US, according to Reuters analysis of court records. At least 250 lawsuits triggered by the coronavirus pandemic have also been filed, according to Loyola Law School professor Justin Levitt, who has been tracking the litigation.
Myrna Perez, who directs the voting rights and elections programme at New York University’s Brennan Center for Justice, said: “In the past, long lines would be disenfranchising or deterring, but in this case they can be deadly.”
Democrats are generally seeking to ease restrictions on mail-in ballots, while the Republicans say they are trying to prevent illegal voting.
:: Subscribe to Divided States on Apple podcasts, Google Podcasts, Spotify, and Spreaker
Both sides have assembled vast legal teams to advise the two campaign teams on potential challenges, particularly in the key swing states.
In many states – including battleground states Michigan, Pennsylvania, Wisconsin and North Carolina – officials will count ballots that arrive after 3 November as long as they were postmarked by election day.
But whether or not the ballots are counted depends on how local election workers enforce rules, notify voters and whether they allow errors to be fixed.
All 50 states have a central election authority, but the ballots are processed by dozens of separate county or municipal election offices within those states.
Most require a signature and witness to be valid. But some states will allow voters to correct a lack of signature or witness, by signing an affidavit. How that is applied can be decided on a local level, and how far officials go to contact the voter is again up to local decision-making.
Both the Republicans and Democrats have begun legal action in key states to try and extend mail-voting deadlines, or to cut them back.
What about fraud?
Mr Trump has repeatedly made claims that mail-in voting will lead to widespread voter fraud, describing the election as “rigged” or a “scam”.
There is very little evidence that the use of mail-in voting does increase fraud, and Mr Trump has yet to provide any.
But the FBI and Department of Homeland Security cybersecurity agency have issued a series of advisories in recent weeks warning voters about potential problems during the election, as well as steps to counter the threat of foreign interference.
11,00 North Carolina residents get incorrect voter registration forms. 2000 LA County Voters received “faulty” Ballots, with NO WAY TO VOTE FOR PRESIDENT. Many others throughout USA. Here we go. This will be the most corrupt Election in American History!
— Donald J. Trump (@realDonaldTrump) October 7, 2020
These issues range from the spread of online disinformation about the electoral process – including deadlines for mail-in votes and drop box details – to cyber attacks targeting election infrastructure.
A nine-minute video message from national security officials has also been released to sooth concerns over mail-in ballots. US officials have revealed ongoing efforts to interfere in the election, including Russian attempts to denigrate Joe Biden.
Will it all end up in court?
Mr Trump has said as much, claiming the result of the 2020 election could be decided by the Supreme Court and refusing to say if he will accept it.
Mr Biden has said he will accept the result.
The death of Ruth Bader Ginsburg last month has triggered a fierce political battle over a vote on her nominated replacement.
The president has insisted the Republican Party should replace Justice Ginsburg as quickly as possible to ensure a ninth member is on the Supreme Court. He has nominated Amy Coney Barrett.
He said: “I think it’s better if you go before the election, because I think this scam that the Democrats are pulling – it’s a scam – the scam will be before the United States Supreme Court.
“I think having a 4-4 situation is not a good situation, if you get that.”
Please use Chrome browser for a more accessible video player
If a successor is appointed by the Republicans, it would almost certainly push the court – which already had a 5-4 conservative majority – more to the right.
That would give the Republicans a 6-3 advantage and Democrats are concerned would ensure Mr Trump won any dispute over the election that ended in the Supreme Court.
In 2000, the Supreme Court’s decision to stop the Florida recount handed the presidency to Republican George W Bush. That is the only time the court has decided the outcome of a US presidential election.
Mitt Romney, Utah senator and former Republican presidential nominee, has criticised Mr Trump for refusing to commit to a peaceful transfer of power.
He tweeted: “Fundamental to democracy is the peaceful transition of power. Without that, there is Belarus.
“Any suggestion that a president might not respect this constitutional guarantee is both unthinkable and unacceptable.”
Mr Trump has also called for the federal court system to ensure that the winner of the election is called just hours after the polls close – despite rules allowing mail-in ballots to be counted several days after the election.
“Now we’re counting on the federal court system to make it so that we can actually have an evening where we know who wins,” he said.
The Democrats have warned for months that Mr Trump will try to “steal” the election by challenging results in the battleground states on voter fraud claims.