In short, the Constitution does not cover dates, or methods, etc., so it is up to Congress, I suppose, to put the foot down: if anyone has proof of voting, postmarks, whatever, by the end of polls closing in a particular state, the VOTE COUNTS, as it pertains to mailed in ballots, so long as it arrives within a reasonable time after polls close, which Congress can define. It could be a week, three days, whatever, we just can’t have each state set arbitrary rules. It is Congress’s responsibility to clear the air.
Nothing violates the RIGHT TO VOTE more than refusing to count a vote cast, by whichever way, BEFORE POLLS CLOSE!
Alito said that the courts may be violating the Constitution, but he has nothing to stand on because nowhere in the Constitution is there a time limit defined for vote counting.
For understanding all the instances where the right or privilege to vote is mentioned:
http://www.theatlantic.com/national/archive/2013/08/what-does-the-constitution-actually-say-about-voting-rights/278782/
We should be looking for an across the board solution, not a jambalaya of arbitrary rules. The SCOTUS even cannot define that, only CONGRESS with a LAW!