[Anitius Severinus Boethius]

The times, places and manner of holding elections for Senators and Representatives, shall be prescribed in each state by the legislature thereof

The "times, places and manner" are determined by the legislatures of each state.

but the Congress may at any time by law make or alter such regulations, except as to the places of choosing Senators.

The Congress may alter the rules (note the exception for the place of choosing Senators. Senators had always been a State concern until the 17th Amendment) but the primary responsibility is the individual states.

But Article I Section 4 specifically covers elections of Representatives and Senators, and not the President.

[Jacquerie]

I’m not sure what you were getting at, but Motor Voter cannot trump the Constitution. Each State determines, subject to the Constitution, the qualifications of Congressional electors.