Even if Joe Biden seemingly triumphs on November 3, the question of who will be US president may remain unanswered, with Donald Trump having raised the prospect of a legal battle.
Winning the last election did not stop Mr Trump from questioning its legitimacy, and the nation being in the grip of the coronavirus pandemic only increases the likelihood he will do so again.
Unless the Democratic challenger secures a resounding victory, Mr Trump will consider taking to the courts in refusing to leave the White House quietly.
– What has the president said?
The Republican incumbent has refused to commit to a peaceful transition of power if he loses the election, in a move that would defy more than two centuries of precedent.
“Well, we’re going to have to see what happens,” Mr Trump told a White House press conference when asked about what he would do win, lose or draw.
He said the “ballots are a disaster” and, pressed over rioting in the States, he added: “Get rid of the ballots and you’ll have a very peaceful – there won’t be a transfer, frankly, there’ll be a continuation.”
Mr Trump has warned of a “rigged election” in what many have interpreted as him trying to delegitimise a result that polling has consistently indicated will not go in his favour.
And he has accused the Democrats – without evidence – of using “Covid to steal an election” and centred a lot of his criticism on postal voting.
– What’s up with postal voting?
Mr Trump has claimed – falsely – that there have been “big problems and discrepancies” with mail-in ballots as he pressed for a final total of the count on November 3.
But some states will continue to count mailed votes for days, and many see his warnings as an attempt to sow distrust in the election’s integrity.
With the Democrats dominating postal voting amid safety fears of travelling to the polls during the pandemic, critics believe the president is setting up a potential challenge on this front.
Mr Biden’s campaign is braced for the Republican to use the so-called “blue shift” in votes, where the Democratic tally increases as postal ballots are counted, to allege the election was being stolen by fraud.
There have already been Supreme Court challenges this election over postal ballots in Pennsylvania and Wisconsin, but any major challenge is likely to be over some nuance that is not yet clear.
– How could the result be challenged?
Mr Trump, in September, said it himself: “I think this will end up in the Supreme Court.”
“I’m counting on them to look at the ballots, definitely,” he later added.
In the wake of Justice Ruth Bader Ginsburg’s death, Mr Trump spoke of needing a replacement to bring the number of justices sitting in the court to nine so no tie can be possible.
– What’s the balance of the court?
That ninth justice, Amy Coney Barrett, is the third to be picked by Mr Trump.
That appointment last month, particularly controversial as it came so close to the election, swayed the conservative balance on the court to six justices against three.
Any decision down party lines, however, would risk further politicising the court and a key figure to watch would be chief justice John Roberts, who has sided with liberals on controversial issues.
– Has the Supreme Court intervened before?
Yes, and the judges were key in the outcome of the highly contested 2000 election between George W Bush and Al Gore.
The race was so close that it came down to the result in Florida, where the pair were separated by just a few hundred votes.
Bitter legal battles ensued and a recount saw rows over the validity of “hanging chads”, punch-card ballots that had not been fully perforated.
But a controversial Supreme Court ruling ended the recount, five to four in favour of Mr Bush, who won the election when his Democratic opponent conceded defeat.
Any legal battle this year could make that one look like a playground scrap, with feelings having reached such a fever-pitch that armed militias are a threat on America’s streets.
– What does the Democratic challenger say?
Mr Biden has said his opponent’s comments on handing over power were “irrational” and his campaign is preparing for any “shenanigans” from the president.
Hillary Clinton has warned Mr Biden not to give up the fight for the White House regardless, having herself lost to Mr Trump in 2016 despite winning the popular vote.
“Joe Biden should not concede under any circumstances because I think this is going to drag out, and eventually, I do believe he will win if we don’t give an inch and if we are as focused and relentless as the other side is,” she said.