In refusing to say whether he would accept the result of the presidential election Donald Trump plunged America into uncharted territory.
It is one of the basic foundations of American democracy .
But, asked if he would abide by tradition, Mr Trump said: I will tell you at the time. I’ll keep you in suspense.
He said the media had “poisoned the minds of the voters” and the election would be “rigged” and riddled with fraud.
A refusal by Mr Trump to accept defeat would effectively give licence to his supporters to cause unrest should he lose.
Already, some of his most fervent followers are ready to do that.
One Trump supporter in Cincinnati told the Boston Globe: “I hope we can start a coup. We’re going to have a revolution and take them out of office. There’s going to be a lot of bloodshed.”
Hillary Clinton called his stance “horrifying”. The Republican National Committee immediately disavowed it. So did his own campaign manager.
Others accused him of being anti-American, unpatriotic, and dangerous.
But in practice what would actually happen if Mr Trump refused to concede?
There is nothing written down, no legal requirement, that he has to. It is only a custom, but one that has always been kept.
The process of Mrs Clinton becoming president would go ahead as usual without Mr Trump’s concession.
But the institution of the presidency itself would be worryingly undermined.
Mr Trump would effectively be setting himself up as an alternative leader for his own supporters.
They may then refuse to recognise the authority of the occupant of the White House.
According to Scott Farris, author of “Almost President: The Men Who Lost The Race But Changed The Nation”, a presidential concession speech is “one of the things that makes American democracy work. Our democracy is a bit more fragile than we think”.
If the election came down to a small number of votes in a few key states Mr Trump could also mount a legal challenge.
But if he is far behind then he will have no realistic recourse in the courts.
His supporters attempted to raise a comparison with Al Gore who contested the 2000 election in the courts in Florida.
But Mr Gore was involved in a razor-thin result and actually won the popular vote.
He also ended up giving a particularly powerful concession speech amid fears his challenge was undermining democracy.
A refusal to accept the election result is something Mr Trump has been talking about since August.
Back them an ally said: “I can’t really picture him giving a concession speech, whatever the final margin. Any poll that shows him behind he thinks it’s rigged.”
Now, Mr Trump declares on a daily basis that he will be the victim of “widespread voter fraud” with illegal immigrants and “dead people” casting votes.
There is no evidence for such an allegation. A study by the Loyola Law School found only 31 credible cases of voter fraud among more than one billion votes cast in elections since 2000.