[M]ust be guided by two cornerstones of [theSupreme Court’s] pre-emption jurisprudence. First, the purpose of Congress is the ultimate touchstone in every pre-emption case. . . . Second, [i]n all preemption cases, and particularly in those in which Congress has legislated . . . in a field which the States have traditionally occupied, . . .[courts] start with the assumption that the historic police powers of the States were not to be superseded by the Federal Act unless that was the clear and manifest purpose of Congress.
We stress that the question before us is not, as Arizona has portrayed, whether state and local law enforcement officials can apply the statute in a constitutional way. Arizona’s framing of the Salerno issue assumes that S.B. 1070 is not preempted on its face, and then points out allegedly permissible applications of it. This formulation misses the point: there can be no constitutional application of a statute that, on its face, conflicts with Congressional intent and therefore is preempted by the Supremacy Clause.(emphasis added)