While federalism justifies variations among state laws, federal criminal law is supposed to be a uniform national response to crimes of national import. On paper, a single set of federal criminal statutes and Federal Sentencing Guidelines applies uniformly throughout the United States. But in practice, federal criminal charges and sentences vary greatly from state to state and from district to district. For example, some districts regularly prosecute low-level drug offenders. Others set high drug-quantity thresholds for charging and refer less significant cases to state authorities. In some districts, defendants must go to great lengths to earn cooperation discounts at sentencing. In others, much less cooperation will suffice.

Some of these variations reflect legitimate local responses to local crime patterns, needs, knowledge, and concerns. Other variations reflect local hostility to national policy choices, methods, and values. The law must accord some weight to local needs, concerns, and limitations, while still ensuring horizontal equity and consistency with national policy. This problem exemplifies the enduring tensions between ex ante rules and ex post discretion, between equality and individualization, and between a synoptic bird's-eye perspective and localized knowledge...