Constitutionally, the Federal government has oversight of elections. While Congress may not tell California how they accomplish the voting, they can create a law of expectations of when all the votes must be counted. Congress can also adjust election law to allow for mail in ballots to be in hand on election day and none can arrive to be counted after.
The Elections Clause The Elections Clause in Article I, Section 4, Clause 1 of the U.S. Constitution establishes a framework for elections, delegating initial authority to state legislatures while granting Congress the power to adjust these rules. States decide the "Times, Places, and Manner" of...
www.usconstitution.net
And I will add that mail-in ballots are likely here to stay, but ID/signature verification...there's a lot that can be tightened up.