I have never seen in my lifetime or in modern political history, any Presidential candidate trying to discredit the elections and the election process before votes have even taken place. There is no serious person out there who would suggest some how that you could even rig America's elections, in part, because they are so decentralized, and the numbers of votes involved.
There's no evidence that that has happened in the past, or that there are instances in which that will happen this time. And so, I'd invite Mr. Trump to stop whining and go try to make his case to get votes. And by the way, doesn't really show the kinda leadership and toughness that you'd want out of a President.
If you start whining before the game's even over, If whenever things are going badly for you and you lose, you start blaming somebody else, then you don't have what it takes to be in this job.