I'm really confused about this because I don't understand the incentives politicians have to adopt this in their own state. It clearly undermines their state's power in national elections. My (cynical) assumption is that this will be obeyed insofar as it helps bring about the desired outcome by those in power. It will be disregarded if it would shift the outcome in the other direction.
Remember, this will work both ways - eventually we will have a democrat that doesn't win the popular vote as well in the current system.