United States Constitution, Article I, Section 4, Clause 1:

    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 chusing Senators.

If you're in a contested state, worried about election day interference/intimidation from ICE (SS) or the Poor Boys (SA), and really want to mail in your ballot, you could do so and still show up a the polls on election day to make sure they've got you down as already having voted.

A good thing for states to do would be to revamp their systems a little bit such that a person can mail in or dropbox a ballot, but still vote at the polls anyway and have their poll vote counted instead of what they've already mailed in. That would protect against outright tampering.

If people do no more than wag fingers, he gets his way.

ita tyranni vincunt