Biden edges it
If Joe Biden wins by a small margin then the result will probably not be known on election night. This election is different due to the scale of early voting, by post and in-person, which has been supercharged by the pandemic.
In some key states officials will not start counting the massive "early vote" until election day itself. One of the key counties in Pennsylvania, perhaps the most key state, will not even start counting the early ballots until Nov 4.
Another Pennsylvania county says it might take them a week to count because they have never dealt with so many before. In addition, in Pennsylvania and North Carolina, delayed postal ballots arriving several days late can still be counted.
If Biden's lead is narrow then he may have to wait days for it to be confirmed. Pollsters say it does not even have to be that close for this scenario to play out.
They estimate if Biden is ahead two, three, or even four points in the nationwide popular vote, then early ballots cast in states like Pennsylvania could still be needed to determine that he has won the Electoral College, and the presidency.
Too close to call
If it's close then Donald Trump or Joe Biden will start asking for recounts in key states.
Then lawyers will become involved as each side disputes what votes should, or should not, have counted.
In the most famous such case George W. Bush and Al Gore fought over "hanging chads," the holes punched in Florida ballot papers, after the close presidential election in 2000.
This time, with so many voters using postal ballots for the first time, the battle could be over something to do with those.
In addition, if the result rests on Pennsylvania, the US Supreme Court has left open that it may rule in the days after the election on whether the state can accept postal ballots three days late.
Overruling that would favour Trump, as Democrats are more likely to vote by post. Already, hundreds of legal cases over election procedures have been filed by Republican and Democrat lawyers.
Whichever cases end up reaching the Supreme Court, Trump would have the upper hand, having appointed three of the judges. The court has a 6-3 conservative majority.
Meanwhile, shops across the US are boarding up in case no result leads to civil unrest.
Florida, one of the key states, is already counting votes that have been cast early. That means the Sunshine State is expected to provide a result on election night, perhaps within a few hours of the polls closing.
It will give everyone, including Trump and Biden, an early indication of how things are going. The electoral map is such that it is, not impossible, but hugely improbable that Trump can win the Electoral College without holding Florida.
As Biden put it recently: "If Florida goes blue, it's over.' In 2016 Trump narrowly won Florida and its 29 Electoral College votes. A quick victory there for Biden, particularly if it is by a clear margin, would indicate a possible landslide.
Texas, which has seen unprecedented early voting, is also geared up to have those ballots, and the ones cast on election day, counted within hours. If Biden wins the Republican bastion of Texas then a landslide is very possible.
A landslide is taken by election observers to be a victory by over 10 per cent in the popular vote, or an overwhelming majority in the Electoral College. A Biden landslide would leave Trump with no options to challenge the result in the courts.
Look out for the "red mirage". Under this scenario states declaring the results of election day results first may show Trump well ahead, as Republicans are more likely to vote in person on the day. As postal ballots are then counted Biden would be expected to catch up.
However, if Trump has a huge initial lead he may well be tempted to declare victory early, while also casting aspersions on the legitimacy of postal ballots. As he trails by a lot in the polls a Trump victory would probably be narrow, and he may ultimately have to wait weeks for each state to certify its results for his win to be confirmed.
Trump's campaign has gamed out numerous scenarios involving different states to get him to the winning line of 270 Electoral College votes but, ultimately, they know he cannot do it without Pennsylvania, and he will campaign there the day before the election.
An average of the national polls has Biden eight points ahead. If Trump wins then the polls will have been more wrong than ever before.