The Criminal Justice Legal Foundation has filed an amicus brief in United States v. California, USDC ED Cal. No. 2:18-cv-00490.  Here is the Summary of Argument:

The Privileges or Immunities Clause of the Fourteenth Amendment has been construed very narrowly by the Supreme Court, but it has not been stripped of all meaning. Among the privileges that the Supreme Court has found included is the duty and right of every citizen to assist federal law enforcement and to give information regarding violations.

By the same principle, citizens have a right and duty to ensure their own compliance with federal law. Because the contours of the law are often uncertain and underlying facts are often unclear, ensuring compliance sometimes means going further than the measures that may, in retrospect, be found to meet the bare minimum for compliance. A state law that punishes citizens for doing anything more than the bare minimum required by federal law therefore, in practice, impairs the ability of the citizens to ensure their own compliance.