From our Nation's founding, the American constitutional order has been a Federal system, ensuring a strong role for both the national Government and the States. The Federal Government's role in promoting the general welfare and guarding individual liberties is critical, but State law and national law often operate concurrently to provide independent safeguards for the public. Throughout our history, State and local governments have frequently protected health, safety, and the environment more aggressively than has the national Government.President Obama, Justice Department's Brief in suit against Arizona's immigration law, July 6, 2010:
An understanding of the important role of State governments in our Federal system is reflected in longstanding practices by executive departments and agencies, which have shown respect for the traditional prerogatives of the States. In recent years, however, notwithstanding Executive Order 13132 of August 4, 1999 (Federalism), executive departments and agencies have sometimes announced that their regulations preempt State law, including State common law, without explicit preemption by the Congress or an otherwise sufficient basis under applicable legal principles.
The purpose of this memorandum is to state the general policy of my Administration that preemption of State law by executive departments and agencies should be undertaken only with full consideration of the legitimate prerogatives of the States and with a sufficient legal basis for preemption. Executive departments and agencies should be mindful that in our Federal system, the citizens of the several States have distinctive circumstances and values, and that in many instances it is appropriate for them to apply to themselves rules and principles that reflect these circumstances and values.
In our constitutional system, the power to regulate immigration is exclusively vested in the federal government. The immigration framework set forth by Congress and administered by federal agencies reflects a careful and considered balance of national law enforcement, foreign relations, and humanitarian concerns -- concerns that belong to the nation as a whole, not a single state. The Constitution and federal law do not permit the development of a patchwork of state and local immigration policies throughout the country. Although a state may adopt regulations that have an indirect or incidental effect on aliens, a state may not establish its own immigration policy or enforce state laws in a manner that interferes with federal immigration law.(via Washington Examiner)
The State of Arizona has crossed this constitutional line.