United States versus Lopez concerned a young student named Alfonso Lopez who brought a handgun to his high school and he was arrested and charged with violating the gun-free school zone act which was a new federal law that prohibited the possession of guns within a thousand feet of a school. So the constitutional challenge was whether the gun-free school zone act was within the enumerated powers of Congress to enact. Lopez concerned the scope of the Substantial Effects Doctrine, which was adopted by the Supreme Court in the New Deal era in the US v. Darby case and then in Wickard v. Filburn.
Both invoked the Necessary and Proper Clause as the constitutional justification for Congress going beyond regulating commerce between one state and another into a state and regulating activities that were neither commerce nor interstate. So what the Substantial Effects Doctrine was was a doctrine interpreting the word necessary in the Necessary and Proper Clause. This becomes important because until Lopez it was basically assumed that Congress's power under the Necessary and Proper Clause was virtually unlimited.
Lopez was a really big case and made a lot of waves. It was a five to four decision with five justices voting in the majority to find the gun-free school zone act to be unconstitutional and four judges in dissent. In his majority opinion Chief Justice Rehnquist limited the substantial effects doctrine to economic activity. Congress could only reach intrastate economic activity because in the aggregate it had a substantial effect on interstate commerce.
Congress could not reach wholly intrastate activity that was non-economic in nature. All the justices agree in principle that Congress's enumerated powers are limited. But this was the first time that the Supreme Court in 60 years had found that Congress had exceeded that limit. The dissenters would not have found that Congress had exceeded that limit.
However, as the majority pointed out, the dissenters could not come up with a single example of an action or law that Congress might pass under its commerce power that would exceed its proper limits. In other words, the dissenters'position had no limiting principle. It was thought at the time, and is often taught since then, that the Rehnquist Court...
was attempting to roll back the powers of Congress that the Supreme Court, during the New Deal, had said that it had. In fact, I think this is a misconception of what the Rehnquist Court was doing. Essentially, it was saying that all the decisions that had happened up until 1995 represented the high watermark of congressional power.
But the Gun-Free School Zone Act was an attempt to go beyond that high watermark, and that is where the court drew the line. And from then on in, advocates would have to present a judicially-administrable limiting principle if they were going to succeed in arguing on behalf of Congress's power.