If you're looking for a specific statute, there it is.
The Controlled Substances Act prohibits physician dispensation of drugs for medically illegitimate purposes. It is a federal law, which means that its execution in the lives of the citizens of the nation is the responsibility of the federal government. Attorney General Ashcroft bears the weight of that responsibility and has rightly made the judgment that physicians cannot dispense federally controlled substances in order to end the lives of patients.
The wording of the statute implies that there must be some judgement taking place. For who is to decide what is or is not legitimate? Congress could do it, but didn't. If they did, unless they frame it in such a way that it is consistent with the just principles of good government, they are in effect usurping the authority of the Creator. That is at least partially how the Declaration is valuable to us, and it is the wisdom of the Declaration that Dr. Keyes reflects on to demonstrate how Mr. Ashcroft's decision was just and good.
Coupled with the authority granted to the federal government by the Constitution (and its duty according to the Declaration) to protect, if necessary and proper, innocent human lives, Atty. Gen. Ashcroft has all the authority he needs. This federal court blew it. I hope the Attorney General and the President continue to pursue it, and that they call on both the Constitution, federal laws, and the Declaration to justify their actions.
The Declaration of Independence has no more to do with allocating duties or powers to the Federal Government than does the Gettysberg Address, the novel Tom Sawyer, or Bill Clinton's first inaugural address. In this country, we have chosen to organize our Federal Government and to limit its powers by way of a written Constitution. If you feel that the states have become too powerful and that the Federal Government is without adequate powers and influence, the Constitution even includes procedural means (Article V) by which it can be amended.
In this particular case, the court found that Congress had not authorized the Attorney General to utilize the Control Substances Act as a Federal weapon with which to interfere with Oregon's assisted suicide law. In that regard, the judge pointed out that in 1998 and again in 1999, the Congress considered amending the Controlled Substances Act so as to enable the Attorney General to use it as a weapon against assisted suicide laws, but that on both occasions the efforts failed. Rather than again seeking Congressional approval, Attorney General Ashcroft decided that he didn't need (read couldn't get) any additional or specific Congressional approval and so he just declared that he already had Congressional approval and proceeded on his own to disrupt the operation of Oregon's law. The judge simply ordered him to stop.
This is not a case of the "state authorities" dictating to the Federal Government "the interpretation of federal law." Attorney General Ashcroft's efforts have been thwarted by an "interpretation of the federal law" by a member of the Federal Government's own judicial branch.