From Alberto Gonzalez' confirmation hearing:

Gonzales: There is no requirement, of course, upon state and locals to enforce federal immigration laws. It is purely voluntary. In fact, of course, some states have prohibitions [against?]. They couldn't, even if they wanted to. [They couldn't what?] In some cases, the department, as I understand it, has entered into with state and local departments in terms of memorandums of understandings in order to enforce this [?]. I certainly am sensitive to the notion that some local law enforcement people don't want to exercise this authority. Well, we're not saying that they have to. If they want to they can assist in fighting the war on terror, that's what this opinion allows us to do. Personally I would worry about a policy that permits someone, a local law enforcement official, to use this authority somehow as a club to harass uhh they might be unlawful aliens but otherwise lawful citizens. That would be troubling. That would be troubling to the President.

He continued: "uhhh... it wouldn't be compassionate... yeah, that's it, it wouldn't be compassionate... they're, uhhh, kind-hearted people... they're here to do the jobs Americans won't do... we're working with our friends in Mexico to end illegal immigration... uhhh, we need a guest worker plan, but it's not an amnesty... I repeat, it's not an amnesty... patroling the borders is hard work... it's hard... we need to be, uhhh... compassionate..."