Eighteen-year-old Alfonso Lopez Jr. was just six weeks short of his high school graduation when, in March 1992, he was caught carrying a .38-cal. handgun on school grounds. Although Lopez insisted that the weapon had simply been given to him by one classmate with instructions to deliver it to another, the San Antonio, Texas, senior suddenly found himself in deep trouble. He had no money for bail, no funds to hire a lawyer and, thanks to a law Congress had passed two years earlier that banned guns within 1,000 ft. of a school, little chance of escaping a six-month prison sentence.

Little chance, that is, until Lopez's court-appointed defense attorney decided that his client's case was hopeless enough to warrant a bold gamble. In a move that is about the closest an attorney ever gets to throwing a Hail Mary pass in the final two seconds of the Super Bowl, Jack Carter conceded that Lopez had indeed broken the law-and then went on to argue that the law itself was wrong. Thirty-seven months and two appeals later, the obscure public defender persuaded five Supreme Court Justices to overturn a law that several hundred legislators and the President of the U.S. had already approved.

What's more, he did it with a constitutional argument that calls into question several decades of federal legislation. Because the majority ruling in U.S. v. Lopez represents a fundamental break with the way the court has interpreted the delicate balance between state and federal authority for the past 60 years, "it may be the most important case of the decade," says Glenn Reynolds, a law professor at the University of Tennessee. "It redefines the nature of the Federal Government."

Contrary to the impression often conveyed by formidable law-enforcement agencies like the FBI, the Federal Government's dominion is limited to only those powers specifically spelled out in the U.S. Constitution. Remaining areas of authority, such as law enforcement and education, are jealously cloistered behind the parapet of state sovereignty. Since the New Deal, however, the Supreme Court has permitted the government to breach certain portions of this wall, using a clause in the Constitution that grants Congress the power to "regulate commerce among the several states." With the court's acquiescence, federal lawmakers have employed the clause as a way to extend their authority into areas of life as diverse as racial discrimination in restaurants and limits on farm products. Most of the civil rights legislation of the 1960s depends on the Commerce Clause. And in recent years Congress has eagerly seized upon the court's generous reading of the clause as a license to extend its jurisdiction even further, adding to the list of federal offenses crimes such as carjacking and drive-by shootings.

But in last week's ruling, the court repudiated this interpretation for the first time in more than a half-century. Led by Chief Justice William Rehnquist, five Justices agreed with Lopez's lawyer that the Gun-Free School Zones Act of 1990 is unconstitutional, rejecting the government's argument that guns in schools contribute to violence, which in turn hampers students' learning and hurts the economy by making students less productive. The court was deeply divided, however. Justice Stephen Breyer, who called the majority ruling "extraordinary," took the unusual position of reading from the bench a portion of his dissent, which argued that "gun-related violence in and around schools is a commercial, as well as a human problem."

It is tempting to interpret the court's impulse to roll back the authority of the Federal Government as a leap aboard the new conservative bandwagon of devolution. The move, however, has been brewing in the court for years, led by conservative Justices. Because the ruling's impact will depend on the court's inclinations, it may be too soon to say whether the Lopez ruling represents, in the words of Yale law professor Bruce Ackerman, a "constitutional moment" -- an epochal period when the court realigns itself with a deep shift in prevailing political philosophy-or rather, as Ackerman's Yale colleague Michael Graetz quips, a "constitutional minute."

It is equally difficult to predict the case's immediate effects. In addition to inviting challenges to 60 years of legislation based on the Commerce Clause, the decision may prompt a new round of challenges to more recent bills like the assault-weapons ban and the Republican push to put federal limits on state-court damage awards. In his weekly radio address last Saturday, Clinton said he had ordered Attorney General Janet Reno to report within a week on how to make the school gun ban constitutional. In the meantime, Alfonso Lopez is heading to a place where firearms are not a problem. He is joining the Marines. -Reported by Nina Burleigh and J.F.O. McAllister/Washington and Andrea Sachs/New York