The federal government will today file suit against Arizona over its new immigration enforcement law, Fox News’ Major Garrett reports. The government will argue that the Arizona law usurps federal authority.

UPDATE: The Washington Post has more (though they suggest the suit won’t be filed until tomorrow at the earliest):

The lawsuit. . .will invoke for 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. Justice Department officials believe that enforcing immigration laws is a federal responsibility, the sources said. . . .
Secretary of State Hillary Rodham Clinton first revealed last month that the Justice Department intended to sue Arizona, and department lawyers have been preparing their case, said the sources, who spoke on condition of anonymity because the government has not announced its plans. The filing is expected to include declarations from other U.S. agencies saying that the Arizona law would place an undue burden on their ability to enforce immigration laws nationwide, because Arizona police are expected to refer so many illegal immigrants to federal authorities.
The preemption doctrine has been established in Supreme Court decisions, and some legal experts have said such a federal argument likely would persuade a judge to declare the law unconstitutional.

UPDATE II: From a skeptical reader (and appellate lawyer);

I’m an appellate lawyer, and while it would be a stretch to claim any special expertise for myself in preemption, my gut tells me this argument will not fly in any court that tries to follow the law.
I’m certainly prepared to stipulate that the federal government alone has the power to set immigration law and policy, and that any state effort to enact a different policy would be preempted.
But the Arizona law, as I understand it, does no such thing. If anything, it seems to be more concerned with, and faithful to, federal statutory law than the federal executive is.
For the feds to argue that the Arizona law will lead to excessive referrals is frivolous; it would be like the FBI complaining that the Chicago Police are catching too many bank robbers.
Nothing will stop the feds, if they so choose, from ignoring the referrals and freeing everyone who gets picked up. Nothing but well-deserved political embarrassment, anyway.

UPDATE III: The suit has reportedly been filed in a Phoenix federal court.