Today the other shoe dropped: Justice Dept. sues, seeks injunction on Ariz. immigration law.

The lawsuit says the law illegally intrudes on federal prerogatives, invoking as its main argument the legal doctrine of “preemption,” which is based on the Constitution’s supremacy clause and says that federal law trumps state statutes. The Justice Department argues that enforcing immigration laws is a federal responsibility.

But the filing also asserts that the Arizona law would harm people’s civil rights, leading to police harassment of U.S. citizens and foreigners. President Obama has warned that the law could violate citizens’ civil rights, and Attorney General Eric H. Holder Jr. has expressed concern that it could drive a wedge between police and immigrant communities.