Sections 3, 5(C), and 6 of S. B. 1070 are preempted by federal law. Pp. 8–19.
Section 3 intrudes on the field of alien registration, a field inwhich Congress has left no room for States to regulate. In Hines, a state alien-registration program was struck down on the ground that Congress intended its “complete” federal registration plan to be a “single integrated and all-embracing system.” 312 U. S., at 74. That scheme did not allow the States to “curtail or complement” federallaw or “enforce additional or auxiliary regulations.” Id., at 66–67. The federal registration framework remains comprehensive. Because Congress has occupied the field, even complementary state regulationis impermissible. Pp. 8–11.
Section 5(C)’s criminal penalty stands as an obstacle to the federal regulatory system. The Immigration Reform and Control Act of 1986 (IRCA), a comprehensive framework for “combating the employment of illegal aliens,” Hoffman Plastic Compounds, Inc. v. NLRB, 535 U. S. 137, 147, makes it illegal for employers to knowingly hire, recruit, refer, or continue to employ unauthorized workers, 8
S. C. §§1324a(a)(1)(A), (a)(2), and requires employers to verify prospective employees’ employment authorization status,
Cite as: 567 U. S. ____ (2012) 3
§§1324a(a)(1)(B), (b). It imposes criminal and civil penalties on employers, §§1324a(e)(4), (f), but only civil penalties on aliens who seek, or engage in, unauthorized employment, e.g., §§1255(c)(2), (c)(8).IRCA’s express preemption provision, though silent about whether additional penalties may be imposed against employees, “does not bar the ordinary working of conflict pre-emption principles” or impose a “special burden” making it more difficult to establish the preemptionof laws falling outside the clause. Geier v. American Honda Motor Co., 529 U. S. 861, 869–872. The correct instruction to draw from the text, structure, and history of IRCA is that Congress decided it would be inappropriate to impose criminal penalties on unauthorized employees. It follows that a state law to the contrary is an obstacle tothe regulatory system Congress chose. Pp. 12–15.
(c) By authorizing state and local officers to make warrantless arrests of certain aliens suspected of being removable, §6 too createsan obstacle to federal law. As a general rule, it is not a crime for a removable alien to remain in the United States. The federal scheme instructs when it is appropriate to arrest an alien during the removal process. The Attorney General in some circumstances will issue awarrant for trained federal immigration officers to execute. If no federal warrant has been issued, these officers have more limited authority. They may arrest an alien for being “in the United States inviolation of any [immigration] law or regulation,” for example, but only where the alien “is likely to escape before a warrant can be obtained.” §1357(a)(2). Section 6 attempts to provide state officers with even greater arrest authority, which they could exercise with no instruction from the Federal Government. This is not the system Congress created. Federal law specifies limited circumstances in which state officers may perform an immigration officer’s functions. This includes instances where the Attorney General has granted that authority in a formal agreement with a state or local government. See, e.g., §1357(g)(1). Although federal law permits state officers to “cooperate with the Attorney General in the identification, apprehension, detention, or removal of aliens not lawfully present in the UnitedStates,” §1357(g)(10)(B), this does not encompass the unilateral decision to detain authorized by §6. Pp. 15–19.