Let's break down the Supreme Court's ruling on the Arizona immigration law like this:

Institutionally

A near-total victory for the federal government. This is a sweeping affirmation of federal power over immigration law. Arizona, and every other state, can't make new immigration policy by making it a crime to be in the United States illegally. States can't even make laws that interfere with federal immigration policies or principles. The court severely limited state "experimentation" in this area, and future presidents and Congresses will be grateful to the Obama administration for bringing this challenge. Bottom line: There will be no crazy-quilt, patchwork bunch of state laws addressing the problem of illegal immigration in America. The question was settled at Appomattox in 1865.

Politically

A split decision. Arizona Gov. Jan Brewer and other supporters of the law can rightly proclaim that the court upheld their main goal: to use the state police and other state law-enforcement resources to address the crisis of illegal immigration in their communities. Never mind that the court struck down so much else in the law. Never mind that the court remains skeptical that even the one controversial provision that was upheld can actually work on the ground in a way that passes constitutional muster. Right now, Arizona police can get in the game of immigration enforcement, in a limited way. But President Obama can claim real constitutional vindication (see above). And Obama can use the ruling as a whole to rally Hispanic voters to his cause. He fought for them (he can argue). He was on their side right down the line.

Practically

Less than meets the eye. As of today, Arizona police can check the immigration status of every person reasonably stopped in the course of keeping public order in the state. But what can they do with that information? There's no state crime on the books any more. States can't deport people. The feds do that. All the Arizona cops can do is inform federal immigration authorities if they round up an illegal immigrant. And the feds are perfectly free to ignore them. That's a lot of hoo-ha, and a lot of police paperwork, for a pretty trifling payoff. How much effort will Arizona police departments put into enforcement? How will training to avoid the problem of racial-profiling affect enforcement? How long can police detain someone while they check her immigration status (the court is especially concerned about this)? How will Arizona residents change their behavior knowing that they might be asked to prove they are in the country legally? How those questions are answered will determine the practical impact of this decision.

Emotionally

This law was designed to create a class of citizens in Arizona that would get special police attention, to make things so difficult for illegal immigrants that they would deport themselves . By upholding the portion of the law that allows local authorities to demand immigration documents, the Supreme Court of the United States just said it's OK for Arizona to do that. From an emotional standpoint, this decision might deepen the divide among Americans on this issue.

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