The key question is whether the Supreme Court will somehow find a way to twist itself into a pretzel, allowing the federal government to dictate to the states the manner in which elections are conducted, in direct contradiction with the Constitution.

Article 1, Section 4:

Article 1, Section 4 of the U.S. Constitution states: "The times, places and manner of holding elections for Senators and Representatives, shall be prescribed in each state by the legislature thereof; but the Congress may at any time by law make or alter such regulations, except as to the places of choosing Senators"

There it is. Federal supremacy wrt the running of elections. If Congress requires proof of citizenship, or only voting in person, on election day, that is the law of the land.

Chew on this one. Electors for POTUS are to be selected by state legislatures, not "we the people".

From the net sifter:

No law requires a public vote to choose presidential electors. Under Article II, Section 1 of the U.S. Constitution, state legislatures have the exclusive power to decide how to appoint their state's electors. A state legislature could theoretically choose electors itself without holding a popular vote.

Article 2, Section 1, Clause 2:

Each State shall appoint, in such Manner as the Legislature thereof may direct, a Number of Electors, equal to the whole Number of Senators and Representatives to which the State may be entitled in the Congress: but no Senator or Representative, or Person holding an Office of Trust or Profit under the United States, shall be appointed an Elector.

So, if a state legislature decides to cut "we the people" out of voting for POTUS, it can. Say the POTUS is staggeringly unpopular: his policies zuk, he is epically corrupt, a state can change it's procedure so that the electors are selected by the bought weasels of the legislature, rather than "we the people". And, with literally, Billions in bribe money floating around, combined with party control of it's members,it would be a slam dunk. Currently, states with GOP control of both houses of it's legislator, control 295 electoral votes.

As has been observed before, we are seeing, for ourselves, how much the US system of governance has depended on the willing good behavior of the participants.

Steve