Republican candidate Donald Trump has hinted that he may challenge the result of the US presidential election if he loses.
"I'll keep you in suspense," he said when asked, in the final TV debate against Democrat rival Hillary Clinton, whether he would accept the result.
Pressed for a response by moderator Chris Wallace, Mr Trump deflected again, saying: "I will tell you at the time."
His comments were supported by his campaign team, but they go against a long and important tradition in US politics that the loser recognises the result and congratulates his or her rival.
Mr Trump's campaign manager Kellyanne Conway and his adviser, former New York Mayor Rudy Giuliani, said the candidate's stance was not unprecedented.
Mr Giuliani said that while the peaceful transfer of power was important, so too was the "accuracy of the election". He said that if the result was close, the campaign would not accept it.
He compared the situation to the contested result in 2000, when George W Bush beat Al Gore. "Al Gore didn't accept it, did he? He went to court over it," he said.
What happened with Al Gore?
The contest between Mr Gore, a Democrat, and Republican candidate George W Bush in 2000 was one of the closest contests in the nation's history.
The result hinged on the state of Florida, where a narrow margin of victory led to numerous lawsuits and weeks of legal battles, triggering a recount.
The argument reached the US Supreme Court, which put a stop to the recount. The contested votes were eventually awarded to Mr Bush, granting him victory.
Mr Gore, who won the national popular vote by a margin of more than 500,000 but narrowly lost in Florida, finally conceded defeat on 13 December 2000.
"Tonight, for the sake of our unity as a people and the strength of our democracy, I offer my concession," he said.
So what can Mr Trump do?
Mr Trump's options are limited. As with Mr Gore, if the result is too close to call he will have the legal right to contest the outcome and demand a recount.
If he believes that electoral fraud has taken place, he can issue lawsuits against officials in any of the states suspected of wrongdoing.
In the event that Mrs Clinton wins with an overwhelming majority, Mr Trump can refuse to accept the loss but there is nothing he can do to alter the outcome.
But while refusing to concede may not alter the result, it would be considered a jab at the very heart of the US democratic and political system.
Moderating the third and final debate on Wednesday, Mr Wallace said the "peaceful transition of power" following hard-fought campaigns was a fundamental US tradition.
"The loser concedes to the winner and the country comes together," Mr Wallace said.
But refusing to concede could cause trouble
If Mr Trump loses and fails to accept his loss, the implications could reach beyond the courts.
His accusations of a "rigged" election, coupled with a refusal to accept defeat, threatens to prompt unrest among his supporters.
The billionaire Republican commands a loyal following, many of whom believe he is an anti-establishment figure being victimised by the Washington political elite.
He has been accused of whipping up conspiracy theories and inciting violence among his supporters. Last month he sparked a backlash after suggesting "Second Amendment people", or gun owners, could take action against Mrs Clinton.
Last week, at a Republican rally in Cincinnati, one Trump supporter warned of a "revolution" and said there would be "bloodshed" if Mrs Clinton won the presidency.
"We're going to have a revolution and take them out of office if that's what it takes. There's going to be a lot of bloodshed," Dan Bowman told the Boston Globe.
Mr Trump has said that the violence at some of his rallies, including one in Chicago earlier this year, was caused by the Clinton campaign and President Obama.
He has denied using hate speech or playing any part in fostering division.