The United States has sued California because of its status as a sanctuary state. California state and local officials either won't help the federal government round up and deport undocumented immigrants or otherwise refuse to participate in the enforcement of federal immigration laws. The lawsuit effectively accuses California of creating "an open borders system," and strongly denounces this as "a radical, irrational idea that cannot be accepted."
This case against California appears to suffer from a significant legal weakness. The Justice Department is asking the federal courts to strike down several California laws, including parts of the California Values Act of 2017, (CVA) which prohibits law enforcement from providing assistance to federal immigration authorities. CVA also prohibits:
the detention of an individual on the basis of a [federal immigration] hold request";
the "transfer of an individual to immigration authorities unless authorized by a judicial warrant or judicial probable cause determination, or in accordance with" California law;
"providing information [to federal immigration authorities] regarding a person's release date if the information is not otherwise available to the public
The Justice Department claims in the lawsuit that those provisions of CVA violate the Supremacy Clause of the U.S. Constitution by "making it more difficult for federal immigration officers to carry out their responsibilities in California" and by "obstruct[ing] the United States' ability to enforce laws that Congress has enacted." According to the DOJ complaint, the provisions of CVA "impermissibly prohibit even the most basic cooperation with federal officials." The Trump administration's Department of Justice wants to force local police to participate in the administration of federal law.
Aside from running afoul of Jeff Sessions's life long belief in states' rights, the lawsuit is at odds with the opinion of conservative legal theory's giant, Antonin Scalia who wrote in Printz v. United States (1997), that the Federal Government "may neither issue directives requiring the States to address particular problems, nor command the States'
officers, or those of their political subdivisions, to administer or enforce a federal regulatory program."
Jeff Sessions is willing to abandon states' rights arguments under the Tenth Amendment and conservative legal theory just so he can increase an already ridiculously long backlog of people awaiting due process in the Immigration Courts.