The doctrine of "preemption" dictates that, as federal law reigns supreme over state law, states may not enforce laws that frustrate federal policies.
Now, the administration is seeking to block three California laws by arguing that they violate the Constitution and federal law.
California Gov. Jerry Brown (D) responded to Sessions' suit Tuesday evening, calling it a "political stunt". Justice Department officials, speaking to reporters Tuesday, said that violates the Constitution's supremacy clause, which renders invalid state laws that conflict with federal ones. Except for those held for a narrow set of serious crimes, these law enforcement officers can not transfer detainees to federal custody voluntarily. None of the groups favored the state law restricting cooperation with immigration officials, but only the California State Sheriffs' Association was actively opposed and some individual officials voiced support.
The lawsuit was filed as the Justice Department is also reviewing Oakland Mayor Libby Schaaf's decision to warn of an immigration sweep in advance, which ICE said allowed hundreds of immigrants to elude detention.
"At a time of unprecedented political turmoil, Jeff Sessions has come to California to further divide and polarize America", Brown said.
DOJ claims that this forces California employers to be caught between state and federal law.
A senior Justice Department official said department lawyers are still evaluating other places' laws and could bring other lawsuits - although the measures California passed stood out as being especially high-profile and transgressive of what Sessions thought was constitutional.
Carol Leveroni, executive director of the 20,000-member California Peace Officers Association, said they invited Sessions in hopes of gaining clarity on how law enforcement can follow California's sanctuary state law, Senate Bill 54, while still following federal mandates.
To make its case, the DOJ is in part pointing to a ruling on a very different state-level immigration law: Arizona's SB 1070, which was meant to expand local police efforts to find and arrest undocumented immigrants.
"The provisions of state law at issue have the goal and effect of making it more hard for federal immigration officers to carry out their responsibilities in California", Justice Department lawyers wrote.
He added: "If we ever pulled our ICE out, and we ever said, 'Hey, let California alone, let them figure it out for themselves, ' in two months they'd be begging for us to come back".
"The Department of Justice and the Trump administration are going to fight these unjust, unfair and unconstitutional policies that have been imposed on you".
The statement also reiterated some of Schaaf's criticism of the Trump administration.
President Donald Trump's administration sued California on Tuesday over its sanctuary state policies. California has the right as a sovereign state to decide how to allocate resources and can't be required to "divert those resources and compromise its security to enforce federal immigration laws", he wrote.
The Trump administration's legal arguments are similar to those it has made in other cases, he said, and his office is prepared to address them.