Arizona says it acted because the federal government had failed to act & protect it from by illegal immigrants, & America agrees.
Article IV, Section 2 of the U.S. Constitution, also known as the supremacy clause.
As noted in numerous posts, this anticipated action by the Department of Justice is not going to help Democrats in November, regardless of what Obama's advisors may think.
From The Wall Street Journal:
The Justice Department filed a lawsuit Tuesday attempting to block Arizona from enforcing an anti-immigration law on the grounds that the state "crossed a constitutional line" that interferes with the federal authority over immigration.
The law, which the state approved in April and was set to take effect later this month, makes illegal immigration a state crime and requires police to verify the immigration status of people stopped for other alleged crimes.
State lawmakers said they were acting because the federal government had failed to protect the state's border with Mexico from encroachment by illegal immigrants.
The Justice Department's brief says that the Constitution and federal law do not allow "a patchwork of state and local immigration policies" around the country, which it says would disrupt federal immigration enforcement.