The way I've heard that it works with this interstate agreement, is that the state's electoral votes are not awarded to the winner of the popular vote until the total electoral votes of participating states exceeds 269 electoral votes. IOW, the agreement goes live when it will guarantee the election of the national popular vote winner, thus effectively bypassing the electoral college system.
If that were to happen, there would be a challenge as to the constitutionality of the interstate agreement. It should fail, but with the way things have been going, who knows...
I was wondering about this and Publius replied, he knows his stuff:
The National Popular Vote compact doesn’t go into effect until:
Enough states sign on to form a minimum of 270 electoral votes;
Congress approves it, which is a requirement of all interstate pacts.
Then comes a court test as to whether the whole thing violates Article IV/Section 4, in which the federal government guarantees to all states a republican form of government.