The U.S. Justice Department is expected to file a lawsuit challenging the constitutionality of Arizona's law cracking down on illegal immigration, and will seek a preliminary injunction to keep it from taking effect on July 29. Arizona's law requires state and local police, while enforcing other laws, to question a person's immigration status if there is reason to suspect that they are in the country illegally. The Justice Department's lawsuit argues that Arizona's measure infringes on the authority of federal laws through the Constitution's supremacy clause. The filing may also argue that the Arizona law violates citizens' civil rights. How strong is the Justice Department's case against the law, and will it be successful?