As surely as presidential candidates promise to change Washington, nominees for attorney general pledge to uphold the law, not personal policy preferences.

Loretta Lynch, now the 83rd attorney general of the United States, was no different when she made her case to Congress in January. Trying to distinguish herself from her lightning rod of a predecessor, Eric H. Holder Jr., she said the law would be her "lodestar."

In the Supreme Court case Nebraska and Oklahoma vs. Colorado, Lynch has a chance to prove that her statement was more than a confirmation hearing cliche.

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Nebraska and Oklahoma claim that the federal Controlled Substances Act, or CSA, preempts Colorado's marijuana law. Because the case involves one state suing another, it falls within a special category of lawsuits that go straight to the Supreme Court.

Typically, the federal government would be the entity seeking to enforce federal law against a state. But because the Department of Justice under Holder refused to challenge Colorado's law, Nebraska and Oklahoma - neighboring states that say marijuana is flowing across their borders and burdening their criminal justice systems - have taken on the task.

The Supreme Court recently asked the federal government to file a brief explaining its position on the issue, which is expected shortly. From a legal standpoint, the correct opinion is obvious: Lynch must side with Nebraska and Oklahoma.

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Before reading any further, please understand that this is not about marijuana, per se. There is a legitimate debate to be had regarding our national marijuana policy. That debate, however, is irrelevant to the issue of whether the CSA invalidates Colorado's marijuana law.

Under the supremacy clause of the Constitution, when federal and state law clash, federal law wins. Accordingly, the Supreme Court has established that if a state law interferes with congressional policies and objectives, it cannot stand.

That's precisely what we have with Colorado's marijuana law and the CSA. Passed by Congress in 1970, the CSA comprehensively regulates the manufacture, distribution and possession of drugs. A central feature of the CSA is a classification system that separates drugs into five "schedules." Schedule I drugs are the most heavily regulated because they have no accepted medical use and a high potential for abuse. From the inception of the CSA to present, marijuana has been listed in Schedule I.

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It is therefore a federal crime to possess, distribute or manufacture marijuana. It is also a federal crime to own, lease, use or maintain any property for the purpose of manufacturing or distributing marijuana. Violations of the CSA are punishable by imprisonment. And the CSA further provides that all money generated by the sale of illegal drugs, including marijuana, is forfeitable to the federal government.

Notwithstanding the clear terms of the CSA and the equally clear terms of the supremacy clause, Colorado passed a law authorizing the distribution, manufacture and possession of marijuana. Put simply, Colorado treats marijuana dealers like legitimate entrepreneurs.

But the reality is that every state-licensed marijuana dispensary in Colorado is in direct violation of the CSA. Every dollar that Colorado's state-licensed marijuana dispensaries generate is forfeitable under the CSA as criminally derived property.

If states are free to disregard federal laws they don't like, then our entire governmental structure is at risk. What's next? Could a state that doesn't like the federal Clean Water Act pass a law authorizing the pollution of its waterways? Could a state that doesn't like the federal Brady Handgun Violence Prevention Act pass a law authorizing gun dealers inside its borders to sell handguns without conducting background checks? Are congressional enactments simply suggestions that the states may accept or reject at their pleasure? That's not how our system is supposed to work.

Recognizing as much, the Justice Department - under Holder's leadership - successfully argued in the 2012 case Arizona vs. United States that federal law preempted Arizona's controversial immigration law. The situation in Nebraska and Oklahoma should receive the same treatment.

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The framers understood that there would be occasional conflicts between state and federal law. And in the supremacy clause, they provided a clear instruction for resolving such conflicts: Federal law wins. That is true regardless of whether the federal law is bad policy or outdated or draconian. And it is true regardless of whether the federal law aligns with the political preferences of the current presidential administration.

If the law is really Lynch's "lodestar," then she has no choice but to argue that the CSA preempts Colorado's marijuana law.

Zachary Bolitho is a professor at the Campbell University School of Law. Before joining academia he was a federal prosecutor with the Department of Justice. He wrote this for the Los Angeles Times.