Enforcement of the tax on remote sales, however, is difficult. Unless the vendor has a physical location, or "nexus", within a state, the vendor cannot be required to collect tax for that state. This limitation was defined as part of the Dormant Commerce Clause by the Supreme Court in the 1967 decision on National Bellas Hess v. Illinois. An attempt to require a Delaware e-commerce vendor to collect North Dakota tax was overturned by the court in the 1992 decision on Quill Corp. v. North Dakota. A number of observers and commentators have argued, so far unsuccessfully, for a Congressional adoption of this physical presence nexus test.