The reality is people hate the Congress and the POTUS - both of which have been Democrat for the past two years.
The polls all showed Republicans running even or ahead in most of the races, which with the polls' ingrained Democrat biases, one could reasonably expect the Republicans to win the toss-ups, yet they lost almost every one.
I have explained to you how it could have happened, without any "evidence" left behind. Once the votes are in the hoppers, nobody has any clue which vote is whose. The evidence is impossible to obtain, unless someone squeals.