"Arizona law would place a undue burden on their ability to enforce immigration laws nationwide, because Arizona police are expected to refer so many illegal immigrants to federal authorities."
That is the main premise of the federal challenge to the Arizona illegal immigration law.
So, the federal government has completely failed to deal with the problem of illegal immigration, but the failure is spread equally among the states so one state can't act on its own. Now that Arizona has take steps where the Feds haven't, the Feds are going to argue that because they haven't acted, an individual state can't either..................no matter how terrible things are in that particular state.
Now let's think about how the Fed lawyers are going to make their argument.
They will have to sheepishly argue that Arizona just has to back off and accept the situation that has been brought about because of a complete lack of law enforcement on the Federal level.
Not only is this legally laughable, it's political suicide.
Oh, and let's forget the fact that over 60% of the American public sides with the State of Arizona.