Consider this language from the Illinois Constitution:

Every United States citizen who has attained the age of 18 or any other voting age required by the United States for voting in State elections and who has been a permanent resident of this State for at least 30 days next preceding any election shall have the right to vote at such election. The General Assembly by law may establish registration requirements and require permanent residence in an election district not to exceed thirty days prior to an election. The General Assembly by law may establish shorter residence requirements for voting for President and Vice-President of the United States.

From the California Constitution:

A United States citizen 18 years of age and resident in this State may vote.

. . .

The Legislature shall prohibit improper practices that affect elections and shall provide for the disqualification of electors while mentally incompetent or serving a state or federal prison term for the conviction of a felony.

And the New York Constitution:

Every citizen shall be entitled to vote at every election for all officers elected by the people and upon all questions submitted to the vote of the people provided that such citizen is eighteen years of age or over and shall have been a resident of this state, and of the county, city, or village for thirty days next preceding an election.

. . .

The legislature shall enact laws excluding from the right of suffrage all persons convicted of bribery or of any infamous crime.

In Moore v. Harper, the Supreme Court held that the state constitutions can restrain state legislatures when they are exercising authority under the Elections Clause–and the Court often refers to the Presidential Electors Clause, too, even though it was not at issue in the case.

The Court pointed toward a more limited understanding of the context of McPherson v. Blacker (1892): “Our decision in McPherson, however, had nothing to do with any conflict between provisions of the Michigan Constitution and action by the State’s legislature—the issue we confront today. McPherson instead considered whether Michigan’s Legislature itself directly violated the Electors Clause (by taking from the ‘State’ the power to appoint and vesting that power in separate districts), the Fourteenth Amendment (by allowing voters to vote for only one Elector rather than ‘Electors’), and a particular federal statute.”

Now, to the National Popular Vote Compact. The compact requires, “Prior to the time set by law for the meeting and voting by the presidential electors, the chief election official of each member state shall determine the number of votes for each presidential slate in each State of the United States and in the District of Columbia in which votes have been cast in a statewide popular election and shall add such votes together to produce a ‘national popular vote total’ for each presidential slate. . . . The presidential elector certifying official of each member state shall certify the appointment in that official’s own state of the elector slate nominated in that state in association with the national popular vote winner.”

State constitutions often fix voter eligibility, including voter eligibility for federal elections. This is not a problem for congressional elections under the Elections Clause, because separate constitutional clauses say that eligibility to participate in House and Senate elections turns on the ability to participate in state elections for the most numerous chamber of the state legislature–which can be fixed by the state constitution.

For presidential elections, however, the Constitution contains no such clause. It is instead subsumed within the state legislature’s power to “direct” the “manner” of appointing presidential electors.

There was a possible basis before Moore, to the extent one had a robust understanding of the Legislature Thereof Clause of the Presidential Electors Clause, that a state legislature could ignore the state constitution when dictating the rules for presidential elections–including, potentially, voter eligibility. This was never really pressed anywhere, but certainly a possibility.

Now, after Moore, it’s fairly clear that states do not have any such power. And that puts the National Popular Vote Compact in a difficult spot.

The three state constitutional provisions I listed above are just a few of the common ones one might see. Voting is restricted to adults, to citizens, to state residents, to non-felons.

But the “national popular vote” creates difficulties under state constitutions. For one, if a state disenfranchises felons or ex-felons, the state constitution forbids those individuals from voting. A national popular vote total that includes such votes in other states (e.g., Vermont, Maine, District of Columbia) would run afoul of the state constitution. Even more significantly, state constitutions provide that residents are able to vote. The national popular vote total includes millions of non-residents from each state, added to a state’s vote totals.

It does not seem that these state constitutions merely set the floor and permit the legislature by statute to enfranchise more voters, such as those under the age of 18, non-citizens, or non-residents. The precise language may matter (e.g., does the state constitution forbid a class of voters from voting, simply ensure that a set of people are eligible, etc.). But if even one state constitution forbids the state legislature from altering the voting base in presidential elections, the compact might not be able to take effect in that state, and it might then drop the total below whatever threshold would be required for it to take effect.

I’m not sure how this might play out or if there are arguments I’m missing. But at first glance, a pretty robust limitation from the Court of McPherson and the Presidential Electors Clause in Moore v. Harper may add one more complication to the National Popular Vote (and its existing share of legal complications).