This is not a threat just an observation but if the left wins and a large amount of Americans doubt the election results, people will begin using extra-democratic methods of redress.
The radical Left has been contesting the election results of 2016 and using extra-democratic methods of redress, see Antifa.
The Democrats don't need to cheat.
They have enough LEGAL voters to win any national election they want to.
In 2020, one third of ELIGIBLE voters will be non-white.
They consistently vote 75%-80% Democrat.
If the Democrats get 40% of the white vote, the election will be a toss up.
If the Democrats get more than 40% of the white vote, Trump will lose.