I think even today's SCOTUS would have a hard time parsing that one to mean anything else.
I think they'd have a very easy time of it, myself. I have 3 grounds for this. (Please note: I think this law is a poor idea - but I also think it is Constitutional. That is all I'm arguing here.)
1. As of this time, the courts have not found the tobacco agreement to be a compact between the states. Given that precedent, you have a very, very high hurdle to jump before anything becomes a compact. This is the point most likely to be overturned, so I'm just putting it up here. But it is worth noting that this is the current precedent.
2. The legislation is crafted carefully to avoid the compact issue. The states are not entering into any type of formal agreement. Instead, each state is deciding how to allocate their electoral votes, and basing it on national popular vote, rather than their own state's total. They specify that they will only follow this method if enough other states have also apopted this method, but that is still reached independently, rather than as an agreement. It lacks the reciprocity features a compact would have.
3. Finally, the Supreme Court, if they actually follow the Constitution (a frequently dubious assumption) would stay they heck out of this argument to begin with. It is left to the States to allocate their votes. It is left to Congress to determine whether to accept those votes (to decide whether the election was conducted properly, etc.) The Supreme Court has no role to play in this, and they should stay out of it. It is, as a result, left to the politics in the States and Congress. Given that, if enough states did pass laws like this, I have little doubt but that they would stand.
Drew Garrett