Article I, Section 8, Clause 3 of the United States Constitution reads: [The Congress shall have power] "To regulate Commerce with foreign Nations, and among the several States, and with the Indian tribes". The original intent was clear, even if it isn't to the Left portion of the political spectrum, which neither knows what the Constitution is nor cares much about what it was intended to accomplish: The Constitution was written so as to create a stronger Union between the States; one fear was that states would set up trade barriers, tariffs, and so on at the borders to protect their own industry. Thus, Congress was given the exclusive power to do so, so that there would be complete free trade between the states. (I suppose the power was reserved to Congress for the rare instances in which regulations would have to be made, e.g. the blockade of the South during the Civil War, although I imagine most examples aren't all that drastic.)
The problem is, that clause has been much abused in pursuit of the increase in Federal power in the last hundred years or so, to the point that it has been completely removed from its original context. Namely, it has been read to give a positive power to Congress, such that Congress is said to have the power to regulate anything relating (however marginally) to commerce between states, a definition that, it has been seen, takes in pretty much all activity by anyone anywhere.
Well, this morning, thinking of how various cities have been talking of boycotting Arizona due to that state's eminently reasonable new immigration law, I got to wondering: Isn't this exactly what the Commerce Clause was designed to outlaw? Don't states have no power in this matter, and aren't they forbidden from discriminating against trade with other areas? I doubt Congress will rise to the occasion, but it seems to me like it would make a beaut of a Constitutional lawsuit. Just my two cents, from thousands of miles away.