Under threat of possible retaliation by the Trump administration, Gov. Jerry Brown signed landmark "sanctuary state" legislation Thursday, vastly limiting who state and local law enforcement agencies can hold, question, and transfer at the request of federal immigration authorities, reports the Los Angeles Times.

Senate Bill 54, which takes effect in January, has been hailed as part of a broader effort by majority Democrats in the California Legislature to shield more than 2.3 million immigrants living illegally in the state. Weeks before Brown's signature made it law, it was met with swift denunciations from Trump administration officials and became the focus of a national debate over how far states and cities can go to prevent their officers from enforcing federal immigration laws.

Brown took the unusual step of penning a signing message in support of SB 54. He called the legislation a balanced measure that would allow police and sheriff's agencies to continue targeting dangerous criminals, while protecting hardworking families without legal residency in the country.

Legal experts have said federal officials may try to block the law in court to keep it from being implemented. Some doubt such challenges would be successful, pointing to the 10th Amendment and previous rulings in which courts have found the federal government can't compel local authorities to enforce federal laws.