During the campaign, it was kind of hard to picture the specifics of how Trump might pull such a thing off. Alas, it’s getting less hypothetical by the week.
On Trumps Draft National Emergency Order to take over US elections - (the National Emergencies Act (NEA) and the International Emergency Economic Powers Act (IEEPA) to claim authority over voting procedures.) let's just say, any attempt to federalize elections would be considered unconstitutional and illegal.
States have control over how they are done as written in the constitution. Also, if SCROTUS allows it, the "We" are no longer a Constitutional Republic.
If Trump is allowed to move forward with this obvious unconstitutional grab for power, allowed by the Dems on a plater with little public outrage, a failed supreme court intervention resulting in no push-back by any checks and balances, then the US Constitution is a lot weaker than americans would have you believe.