The Department of Justice will file suit against Arizona this week in an attempt to overturn SB 1070. The federal government will base its case on preemption, even though Arizona does not create new immigration violations and bases its statute entirely on existing federal law. But this argument is particularly rich:

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.

It is precisely the federal government’s failure to enforce its own immigration laws — and their incomplete border fencing that has redirected illegal immigrants to Arizona specifically — that would account for such referrals. Federal action would have made the Arizona law unnecessary But the Obama-led feds would rather crack down on Arizona than illegal immigration.