Obama won’t allow it. He’ll declare Martial Law.
If Trump said he wanted a million people in the streets around the white hut it would happen. Think he’ll draw a million in Cleveland?
The thing to worry about is ballot stuffing, not martial law. The president is elected by the states, and by the people only at the discretion of the states. Mess with the voting, and the state legislatures just select Electors directly. Always assuming they dare to do it, which unfortunately is not a given.