That might make their case even harder to win, according to Jack Balkin of Yale Law:
The new Arizona law aims to assist in enforcement of federal laws against illegal immigration. So this is not a statute that only incidentally affects federal immigration policy. Quite the contrary: its stated purpose is to cooperatively assist the federal government in protecting the nation's borders, identifying illegal aliens, apprehending them, and delivering them to federal authorities. The bill was deliberately written to be preemption-proof by tracking federal definitions and placing state law enforcement officials in the service of enforcing federal law.
If so, how can the law be preempted by federal law? The answer is that the federal government might well believe that Arizona's attempt at helping it enforce its immigration laws is counterproductive and therefore actually conflicts with federal enforcement policy. In this case, the fact that Arizona is tracking federal definitions of who is an illegal alien might make things worse for the law's constitutionality, not better. First, Arizona will not be able to justify the law on the grounds that it has only incidental effects on federal immigration policy; Second, it will be more difficult for Arizona to argue that the scope of its new law is not already occupied by the federal scheme and that the law does not interfere with federal law's balancing of the relative costs and benefits of adopting particular enforcement policies.
The leading Supreme Court case on preemption of state immigration law, De Canas v. Bica, is now thirty five years old and predates recent federal immigration reforms, so it is anybody's guess about what the current Supreme Court would think about the issue. De Canas upheld a California law which made it a crime to knowingly hire an illegal aliens; the decision has lots of language that gives states plenty of room to pass immigration regulations consistent with federal law. On the other hand, DeCanas assumed that the law in question did not significantly interfere with federal immigration policy, or at least, there was no evidence in the record that it did.
There is a much stronger argument [than was available in De Canas] that the new Arizona law, while purporting to be helpful, actually sticks a thumb in the eye of the federal government by engaging in draconian measures. The Arizona legislature appears to be saying, in effect: "since you won't police the borders, we will, and if you don't like it, pass some new legislation." If this is the point of the new Arizona law, then the law isn't really an attempt at cooperation but an attempt at provocation and one-upmanship, and the chances that it is preempted increase.