"Sanctuary" policies vary widely. They typically include some combination of: declining to hold immigrants for ICE pickup beyond their criminal release dates ("detainers"), refusing 287(g) agreements that deputize local officers as immigration agents, and limiting information-sharing between local agencies and ICE.
Federal courts have generally held that detainers cannot be made mandatory under the 10th Amendment's anti-commandeering doctrine. Federal grant conditions tied to immigration cooperation have been litigated extensively.
Defenders argue sanctuary policies improve community-police trust and public safety. Critics argue they shield criminals and frustrate federal enforcement.