Well, my Federal Courts class was a long time ago, and I’m not an American, so the details are a bit fuzzy, but since none of our US law Dopers have wandered by I’ll take a stab at it.
Article V establishes that federal laws enacted under the Constitution are supreme over state laws. So, if there is a conflict between a federal law and a state law, the federal law takes priority.
Now, if all the marijuana measures do is de-criminalise the possession of marijuana under state law, I doubt that there’s conflict. It’s just that the federal law makes something criminal that the state doesn’t.
But I gather from this thread that some of the ballot proposals go further, and authorise or require the states in question to provide for medical marijuana. If that’s an accurate summary, I can see why there’s a conflict: federal law prohibits trafficking in marijuana, as an aspect of inter-state commerce. The state law isn’t just silent on the issue of marijuana; it’s telling the state to set up the Medical Weed Distribution Board. That sounds like trafficking under federal law.
If I’ve accurately summarised it, then yes, the feds could bust the Medical Week Distribution Board and its members and employees for trafficking; but that’s bad publicity for the feds; ADT folks in flak jackets arresting state employees on trafficking charges on the 6 o’clock news isn’t likely to garner a lot of support. And, it’s in the feds’ interest not to even let it get so far as the Medical Weed Distribution Board actually being set up and ready for business.
So instead, to try to foreclose it entirely, the federal DOJ can bring an action for an injuncition, in either state or federal court (as AK84 suggested), seeking a declaration that the state laws establishing the Medical Weed Distribution Board conflict with the federal law on trafficking, and therefore trigger the pre-emption doctrine. The feds would also seek a permanent injunction, enjoining the state officials from even setting up or staffing the Medical Weed Distribution Board, because they want to nip this whole thing in the bud.
If the court agrees with that argument, then the state law is rendered inoperative so long as the federal trafficking laws apply to marijuana, and the Medical Weed Distribution Board doesn’t get set up. And, if a state were to defy the court order and tries to set up the Board, it’s a much better P.R. position for the feds - instead of black-flak jacketed ADT thugs, arresting hapless state officials, the feds would sue for contempt, arguing that even if you don’t like the injunction, the rule of law requires all public officials to comply with court orders. That’s a much better argument for the feds to be making on the news.
Wiki has a nice article on Federal pre-emption.
And, as I said, I’m not an American, nor an American lawyer, so this is just my best stab at a reply. Would welcome hearing from a real genuine American law-talking dude.