That is easy. The Fourth Amendment to the US Constitution. The real question here is if that is a reasonable stop or not. Obviously, people in the thread disagree on this point.PsychDiver wrote:Please site where in the Constitution or Bill of Rights were it is forbidden to ask someones there immigration status!
The Fourth Amendment, contrary to popular belief, does not forbid all searches without a warrant. It forbids "unreasonable" searches and seizures. What this means is that the court case will be fought on whether or not the specific type of search is reasonable. SCOTUS is the group with the real power to finally decide this, and each of us gets to make only an interim decision. SCOTUS is supposed to look at how the overall population feels to decide what is reasonable. This is why they will change their minds over time on a specific subject.
As a general rule, courts have always upheld as reasonable a customs search or request for information. Usually, this is done at the border. Thanks to the war on drugs, the courts have come up with the concept of a flexible border. This allowed police to make arrests on packages that were shipped from out of the country to a specific address many miles from any border. This same concept is what allows for a border checkpoint inside the US but close to the border.
I personally have some heartburn with this stop when I have not crossed the border. Given the proximity to the border, the length of the stop and the lack of other questions asked, I have not come to a real conclusion on whether or not the checkpoints are reasonable. I don't like them, and would prefer to have a real secure border, but it is also not a significant (to me) infringement yet. If they tried a checkpoint of this type somewhere else, say on IH-35 north of San Antonio, I would have to say it was wrong. But within an hour drive of the main border entries may not be unreasonable.