We have to always remember how thorough the opposition is in their voter suppression. In fact, there is no other area of governance, certainly not policy, where they explore and exploit every angle and detail to reach their desired end–preventing the “wrong” people from voting.
So, added to the always ongoing Republican voter suppression, it wouldn’t take much in the form of Russian mischief–messed up voter roles here and there or, more likely, the ongoing campaign of disinformation–just enough to push some undecideds into the other camp. Or just enough to pit Progressives against more mainstream Dems.
It was all of it together, a toxic brew.