Congress may be able under the Necessary and Proper Clause to require that state legislatures set the rules for selecting their presidential electors in advance of the election. Congress may also, as the Electoral Count Act purports to do, set dates for the casting of electoral votes in the states, and for the counting of those electoral votes by Congress. But, Congress has no power to tell a state who is eligible to vote in a presidential or state election. (There are many scholars who think the Electoral Count Act is unconstitutional.)

Congress also cannot prevent the states from returning to the situation at the nation’s founding where some state presidential electors are picked by state legislatures. And Congress cannot tell Maine and Nebraska to pick their presidential electors at large, nor could Congress tell state legislatures that they have to award an electoral vote to the winner of each state’s congressional district. To my mind, it would be a very bad idea for state legislatures to pick presidential electors rather than holding a popular vote for president, but it would not be unconstitutional.

What would be clearly unconstitutional, however, is for Congress to set national voting rules as to who is eligible to vote in presidential or state elections. The Constitution leaves that power exclusively to the states.