A federal lawsuit against Arizona's tough new immigration law focuses heavily on a question that dates to the Founding Fathers: the right of the government to keep states from enacting laws that usurp federal authority. "Immigration has traditionally and constitutionally been the historic preserve of the federal government, and there are cases going back to the late 19th century that say as much," said Peter Spiro, an immigration law expert at Temple's Beasley School of Law. "So the Obama Administration has a lot to work with in filing this claim, and the fact that the claim is filed by the administration adds credibility…and increases the chances that law will be struck down on pre-emption grounds."