Yes, the limits of prosecutorial discretion are legally murky. What I'm referring to is the legality of enforcing a valid federal law that conflicts with a state law; there is no question whatsoever that the federal law is supreme and may be enforced.
As for grand-standing state AGs, that's not just a red state thing. A few examples come to mind from recent months alone. Look at the Washington State AG, filing all sorts of clearly losing political lawsuits, e.g. suing a motel for sharing guest-list information with the federal government, on the theory that it violates state consumer protection law. Or look at the various state AG's who are hopelessly suing over the net neutrality repeal. These are losing cases, filed as a red meat for their electoral base.
As for grand-standing state AGs, that's not just a red state thing. A few examples come to mind from recent months alone. Look at the Washington State AG, filing all sorts of clearly losing political lawsuits, e.g. suing a motel for sharing guest-list information with the federal government, on the theory that it violates state consumer protection law. Or look at the various state AG's who are hopelessly suing over the net neutrality repeal. These are losing cases, filed as a red meat for their electoral base.