The employees of the US Treasury Department that work the border crossing between the United States and Mexico learned that they will be subject to routine drug testing. The customs bureau, which is a division of the treasury department, announces this policy along with its reasoning: since customs agents must routinely search for drugs coming into the United States, it makes sense that border guards must themselves be completely drug-free. Many border guards do not use drugs, have no intention of using drugs, and object to the invasion of their privacy. What is the constitutional basis for their objection?

Please assist me so I can study this for my midterm, thanks!