Because the President reserves the right to declare that we are at war - and then he can declare a state of emergency, granting almost unchecked power to the President.
He can then Martial Law the entire country - and create circumstances that end up cancelling the election. If no election happens - the office would be left vacant, according to the Constitution. To which, under that emergency power -- he could declare that he will retain office until time that an election can occur - after the emergency is over and after martial law is complete.
There's no precedent for this occuring in the history of the US, but the potential is there. And if we have learned anything is that Democrats literally will do ANYTHING to keep their power and control over the people.
Caption this Meme
