Art. III, Section 2, Clause 1

There needs to be an actual case or controversy. The litigation likely could come after Congress elects either to approve or disapprove of the compact after the requisite number of states subscribe.

As to your hypothetical, wouldn’t it be difficult to know with any certainty how the candidate would fare so far in advance of a general election?

I also question whether States actually need to enter a compact or agreement to accomplish the goal of the “Compact.” It may be that the initiative just has an unfortunate name.

It’s axiomatic that States have the constitutional right of states to choose their electors: “Each State shall appoint, in such Manner as the Legislature thereof may direct, a Number of Electors . . . ” (U.S. Const., Article II, Section 1, Clause 2).

This, taken with the Compacts clause, would suggest that so long as each State acts unilaterally (i.e., no express or implied agreement), they may individually adopt legislation to award their electors to the winner of the national popular vote without offending the Compact Clause. The fact that States may be acting in parallel doesn’t necessarily mean that there is an agreement or compact, though it perhaps could be circumstantial evidence of one.