	Introduction

While the power to prescribe rules as to which aliens may enter the United States and which aliens may be removed resides solely with the federal government, the impact of alien migration—whether lawful or unlawful—is arguably felt most directly in the communities where aliens settle. State and local responses to unlawfully present aliens within their jurisdictions have varied considerably, particularly as to the role that state and local police should play in enforcing federal immigration law. 1070, were facially preempted by federal immigration law. At the other end of the spectrum, some states and localities have been unwilling to assist the federal government's enforcement of measures that distinguish between those residents with legal immigration status and those who lack authorization under federal law to be present in the United States. In some instances, these jurisdictions have adopted measures that seek to thwart federal efforts to identify and apprehend unlawfully present aliens within the state or locality's jurisdiction. While state and local restrictions on cooperation in federal immigration enforcement efforts have existed for decades, there has been an upswing in the adoption of these measures in recent years. Moreover, the nature of these restrictions has evolved over time, particularly in response to recent federal immigration enforcement initiatives like Secure Communities (subsequently replaced by the Priority Enforcement Program (PEP), as discussed below), which enable federal authorities to more easily identify potentially removable aliens in state or local custody. This report discusses legal issues related to state and local measures limiting law enforcement cooperation with federal immigration authorities. It begins by providing a brief overview of constitutional principles informing the relationship between federal immigration authorities and state and local jurisdictions. The report then discusses various types of measures adopted or considered by states and localities to limit their participation with federal immigration enforcement efforts, including (1) limiting police investigations into the immigration status of persons with whom they come in contact; (2) declining to honor federal immigration authorities' requests that certain aliens be held until those authorities may assume custody; (3) shielding certain unlawfully present aliens from detection by federal immigration authorities; and (4) amending or applying state criminal laws so as to reduce or eliminate the immigration consequences that might result from an alien's criminal conviction. Some states and localities have sought to restrict police cooperation with federal immigration authorities' efforts to apprehend removable aliens. Entities that have adopted such policies are sometimes referred to as "sanctuary" jurisdictions, though there is not necessarily a consensus as to the meaning of this term. Although criticized by some for impeding federal immigration enforcement efforts, state or local policies of declining to honor at least some immigration detainers would appear to be permissible under federal law. Nothing in the INA purports to require that states and localities honor immigration detainers.
While the power to prescribe rules as to which aliens may enter the United States and which aliens may be removed resides solely with the federal government, the impact of alien migration—whether lawful or unlawful—is arguably felt most directly in the communities where aliens settle. State and local responses to unlawfully present aliens within their jurisdictions have varied considerably, particularly as to the role that state and local police should play in enforcing federal immigration law. Some states, cities, and other municipalities have sought to play an active role in immigration enforcement efforts. However, others have been unwilling to assist the federal government in enforcing measures that distinguish between residents with legal immigration status and those who lack authorization under federal law to be present in the United States. In some circumstances, these jurisdictions have actively opposed federal immigration authorities' efforts to identity and remove certain unlawfully present aliens within their jurisdictions.
Although state and local restrictions on cooperation with federal immigration enforcement efforts have existed for decades, there has reportedly been an upswing in the adoption of these measures in recent years. Moreover, the nature of these restrictions has evolved over time, particularly in response to recent federal immigration enforcement initiatives like Secure Communities (subsequently replaced by the Priority Enforcement Program), which enable federal authorities to more easily identify removable aliens in state or local custody. Entities that have adopted such policies are sometimes referred to as "sanctuary" jurisdictions, though there is not necessarily a consensus as to the meaning of this term or its application to a particular state or locality. Recent reports that an alien who shot and killed a woman, after being released by San Francisco authorities who had declined to honor an immigration detainer issued by federal authorities, have brought increased attention to state and local practices of declining to honor such detainers, as well as "sanctuary" policies more generally.
This report discusses legal issues related to state and local measures that limit law enforcement cooperation with federal immigration authorities. The report begins by providing a brief overview of the constitutional principles informing the relationship between federal immigration authorities and state and local jurisdictions, including the federal government's power to preempt state and local activities under the Supremacy Clause, and the Tenth Amendment's proscription against Congress directly "commandeering" the states to administer a federally enacted regulatory scheme.
The report then discusses various types of measures adopted or considered by states and localities to limit their participation in federal immigration enforcement efforts, including (1) limiting police investigations into the immigration status of persons with whom they come in contact; (2) declining to honor federal immigration authorities' requests that certain aliens be held until those authorities may assume custody; (3) shielding certain unlawfully present aliens from detection by federal immigration authorities; and (4) amending or applying state criminal laws so as to reduce or eliminate the immigration consequences that might result from an alien's criminal conviction. For discussion of legal issues raised by states and localities seeking to play an active role in enforcing federal immigration law, see CRS Report R41423, Authority of State and Local Police to Enforce Federal Immigration Law, by [author name scrubbed] and [author name scrubbed].