The first reason the loss at the polls needs to be total: to snuff out the corrosive fiction that the system is rigged. Trump hasn't just leveled this charge, ridiculously, about the Republican primary, which he won despite the elites trying to stop him, and somewhat less ridiculously about the Democratic one. He's now complaining before the fact that the November election will be fixed.
The complaint is code for the fact that Trump, somewhere in his hindbrain, knows that he lacks the power, money, skills and the discipline to win this race. When the polls were breaking his way, our republic was perfect; the voters had spoken and he was "doing very, very well." Now that the worm is turned, the evil elites will do anything to stop him.
As though his demagoguery hasn't been poisonous enough already, now he needs his minions to believe that if and when he loses, it was because some nefarious conspiracy of state officials was sitting in dark rooms literally switching votes from Trump to Clinton.
But it means that, if there's a loss by a slim margin in the popular vote or electoral college, millions of already embittered Americans, worked into a frenzy by a shameless leader who will surely refuse to accept the returns, will start the next four years convinced that the United States of America is little more than a banana republic — and the presidency of Hillary Clinton is irretrievably illegitimate.
That will be awful for the country.
