The third debate has sealed it. He wheezed and whines, insulted and incited, snivelled and sniped, insulted and incited, interrupted and harangued and refused to say that he will accept the outcome of the election. This is dangerous and unacceptable as it questions the very foundation of American democracy.
The past ten days or so have seen a parade of women, claiming to have been sexually assaulted by Trump. They came out in response to Trump's claim, in the 2nd debate that he had never sexually assaulted anyone. They range in age and time and careers and all he could say was that they were to ugly for him "look at her".
Most of the Republican hierarchy have abandoned him, Senators, Representatives, Governors, Party Officials and so he has launched a vitriolic attack against them, saying that there is a conspiracy against him. He blames everyone but himself.
He continues to claim that the System is "rigged" against him. In the 3rd debate, he was asked twice by the host whether he would accept the outcome and all he would say is that he will wait and see. He speaks to the most rabid element of his supporters, the White Supremacists, the KKK, the Militias, the Conspiracy Theorists, the government haters, the anti-immigration lobbyists etc. and in this there is grave danger. This is inciting violence.
There is no precedent for this. In 1824, Jackson accepted defeat when Adams was elected, even though there were legitimate questions; in 1960 Nixon accepted the result, even though there were questions about the vote in Dallas and Chicago and in 200 Al Gore accepted his loss even though Florida was lost by only about 550 votes and there were questions about the count and accepted even the Supreme court split vote.
There is more than an election at stake. The very idea of America has been challenged by Trump.