The Department of Justice filed its long-awaited lawsuit against the state of Arizona today in federal district court. How will Attorney General Eric Holder challenge the state's immigration law, specifically? The case rests on two points:
1) That Arizona's law, SB 1070, violates the Constitution's Supremacy Clause by setting up a state-level immigration-law regime. The Constitution gives the U.S. Congress the power "to establish an uniform rule of Naturalization" in Article I, Section 8; Article VI, the Supremacy Clause, states that the Constitution "shall be the Supreme Law of the Land," above any state law.
2) That SB 1070 conflicts with federal immigration law and enforcement. Immigration laws are drafted, and enforced by DoJ and the Department of Homeland Security, with a host of federal concerns in mind, such as relations with other countries. Arizona's law, which would punish anyone found to be here illegally (aka, unable to supply papers), interferes with the sensitive and, at times, selective enforcement of immigration statute. For instance, the federal government will grant some illegals asylum from persecution, and it will provide special visas to victims of trafficking or violent crimes. Furthermore, federal agencies "principally" target suspected violent criminals in enforcing immigration statute, and the violation of immigration laws, federally, doesn't always necessitate criminal punishment. By requiring everyone to show papers and criminally punishing everyone who doesn't have them, Arizona interferes with those nuances, DoJ argues.
Arizona Gov. Jan Brewer, who is named as a defendant along with the State of Arizona in the suit that was filed today in the district court in Phoenix, has not released a statement, so it is not yet clear what the state's defense will be. Given the political rhetoric Brewer and others have employed, it's probably safe to assume that the federal government's alleged failure to address immigration will play some role in their legal arguments.
You can read the full, 25-page complaint below. The two-page Introduction gives a taste of the Justice Department's argument: