Donald Trump has said the election system is "rigged" and he would only accept the election result on 8 November if he wins.
What would happen if he did not concede the election?
Trump could contest the result and take it all the way to the supreme court, according to New York University’s professor of constitutional law, Samuel Issacharoff.
"He can to court and challenge the outcome of the election," the professor explained.
"He can try to sway individual members of the electoral college to abandon Hillary Clinton in his favour - but that has pretty much never happened in American history."
If the Republican decided to not accept the result, as he has warned, then he would start his case in the federal district court and it would most likely end up in the supreme court.
"It’s [the supreme court] very unlikely to rule in his favour," said Mr Issacharoff.
"The supreme court is not in the business of overturning elections. It would act to ratify the electoral result."
There have only been three contested presidential elections in American history, in 1800, in 1876 and in 2000. Democrat Al Gore contested the 2000 election against Republican George W Bush in Florida and the issue was resolved in the supreme court one month after the election.
In 2016, Trump or Clinton would need 270 electoral college votes to win the election.
The electoral college - the official body that elects the president - convenes in Washington DC in January, when the new president is officially sworn in to the White House.
"I think that if Hillary Clinton gets more than 280 electoral votes, history will pass Donald Trump by with blazing speed," he said.
But Trump, who has spent months inciting his followers to support the idea of Ms Clinton in prison, to "monitor" polls to "prevent voter fraud", there are still thousands if not millions of Americans who would expect Mr Trump to challenge the result if he loses.
"There are some people who still believe the earth is flat," responded the professor.
"We’re talking about marginal figures and if they engage in violence they will be dealt with the appropriate way."
(The Independent)
