Question For Our Law Professors

Professors Yoo and Epstein, if you don’t mind a question from the back of the class, I’d like to shift to a different topic and benefit from your expertise.

Word is the DOJ is preparing to take action against Arizona regarding its new immigration law. Specifically, they are contending that the Arizona legislature exceeded its authority by effectively impeding federal responsibility to enforce its immigration laws. From the perspective of a layman, three questions arise:

First, how in the name of Judge Wapner’s gavel can federal enforcement be impeded when there is no federal enforcement taking place in the first instance? It would be like citing me for impeding traffic when I’m the only one on the road, no? If your answer is that it is the fed’s responsibility, not their enforcement, that is being impeded upon, I would counter that it is a responsibility that is being ignored. Am I wrong? 

Second question: How can the fed’s responsibility in this matter be impeded or infringed when the new state law merely restates existing federal law?

Lastly, do you gentlemen think that DOJ will suceed in its effort to derail the Arizona law?