Read this column by Dana Milbank in the paper today:
This matters, because it means the entire premise of the Arizona immigration law is a fallacy. Arizona officials say they've had to step in because federal officials aren't doing enough to stem increasing border violence. The scary claims of violence, in turn, explain why the American public supports the Arizona crackdown.
I know what he means, but I'm a stickler for such things, and it's wrong to call this a "fallacy." A fallacy is an error in reasoning and Milbank is simply alleging that the factual basis of the law (more on that in a second) is false. Were it to be true, then there would be no fallacy. So they're just mistaken about facts.
As for the allegedly false factual basis, the most Milbank can say is that some of the claims made by various supporters of the Arizona immigration law are false. I don't think that amounts to the claim that the "entire premise of the law" is false. I imagine there are other premises–such as illegal immigration is illegal, and so forth–that supporters of the law can point to.
None of this means, of course, that the law in question is a good idea–it's just not a fallacy.