On March 5, 1976, a Baltimore man named Michael Lee Smith robbed a woman named Patricia McDonough. Smith drove off in a late-model Monte Carlo, which McDonough described to the police. After the robbery, he began making threatening phone calls to his victim. The cops tracked down Smith’s home address by tracing his license-plate number, and they soon set out to prove that he was the same person harassing McDonough over the phone.

Without obtaining a court order or warrant, Baltimore police instructed the local phone company to install a mechanical device known as a pen register at its offices to record the numbers dialed from Smith’s rotary phone. The pen register was a primitive technology. It used paper tape to record the information, and it didn’t actually tell the police whether the attempted phone calls were ever completed. But the register detected that on March 17th, the phone at Smith’s home dialed the seven digits of McDonough’s home phone number.

The information led to Smith’s arrest and prosecution, but his lawyers argued that the key evidence, from the warrantless installation of the pen register, should be suppressed because it violated the Fourth Amendment, which protects Americans “against unreasonable searches and seizures.” The trial court disagreed, and Smith was convicted and sentenced to six years in prison. An appeals court upheld the decision, but with a notable dissent by three judges, who argued that there was indeed an expectation that the phone numbers that Americans call are private, and thus require a warrant before they can be collected.

In 1979, to resolve the conflict, the Supreme Court took up the case. “The installation and use of the pen register was not a ‘search’ within the meaning of the Fourth Amendment, and hence no warrant was required,” the court said in Smith v. Maryland. In language that would become enormously important to the Bush and Obama Administrations after 9/11, the court held that “it is doubtful that telephone users in general have any expectation of privacy regarding the numbers they dial, since they typically know that they must convey phone numbers to the telephone company.”

In a prescient dissent, Justice Thurgood Marshall warned that the Court’s treatment of phone numbers as less deserving of protection than the actual content of phone calls “ignores the vital role telephonic communication plays in our personal and professional relationships” and “the First and Fourth Amendment interests implicated by unfettered official surveillance.” He hinted at a future in which the court’s decision was used to collect phone records on a much larger scale, and warned of troubling implications if it were allowed to do so:

The prospect of unregulated governmental monitoring will undoubtedly prove disturbing even to those with nothing illicit to hide. Many individuals, including members of unpopular political organizations or journalists with confidential sources, may legitimately wish to avoid disclosure of their personal contacts. Permitting governmental access to telephone records on less than probable cause may thus impede certain forms of political affiliation and journalistic endeavor that are the hallmark of a truly free society. Particularly given the Government’s previous reliance on warrantless telephonic surveillance to trace reporters’ sources and monitor protected political activity, I am unwilling to insulate use of pen registers from independent judicial review.

Fast-forward twenty-seven years, to 2006, when Alberto Gonzales, George W. Bush’s Attorney General, was scrambling to find a legal justification for the mass collection of phone records—a program that had been up and running since October, 2001. On May 23, 2006, Gonzales secretly filed a memorandum of law before the Foreign Intelligence Surveillance Court requesting that the major phone companies be formally ordered to hand over the telephone records of all Americans on an ongoing basis.

“There is, of course, no constitutionally protected privacy interest in the information contained in call detail records, or telephony metadata,” he told the court, citing Smith v. Maryland. The next day, the court granted the Bush Administration’s request. After Edward Snowden revealed the details of the program, President Obama made the same argument as Bush’s lawyers. In a “White Paper” that intelligence officials released in August justifying the legality of the program, the Obama Administration argued that “participants in telephone calls lack any reasonable expectation of privacy under the Fourth Amendment in the metadata records generated by their telephone calls.”

Lawyers in both the Bush and Obama Administrations have dismissed the idea that there is any difference between the 1979 case concerning a single pen register, recording phone numbers from one rotary phone for a few days, and the N.S.A. collecting all phone records indefinitely while regularly searching their contents. And indeed, numerous judges at the FISA court have reauthorized the program since Gonzales’s victory in 2006. (As I explained in a recent article, from October, 2001, to May, 2006, the program operated without the FISA court’s authorization, and was probably illegal.)

But yesterday, for the first time, a federal judge said that Smith v. Maryland, which is now thirty-four years old, is irrelevant to the question of whether the N.S.A.’s mass surveillance program is constitutional. Judge Richard J. Leon, the U.S. district judge who is hearing a challenge to the N.S.A.’s program, essentially declared that the facts and circumstances surrounding the use of a pen register on Michael Lee Smith’s phone in 1979 and the N.S.A.’s collection and search of everyone’s phone records in 2013 are so vastly different that it is ludicrous to use the decades-old opinion to justify the N.S.A. program.

“The question before me is not the same question the Supreme Court confronted in Smith,” he wrote. “Indeed, the question in this case can more properly be styled as follows: When do present-day circumstances—the evolutions in the Government’s surveillance capabilities, citizens’ phone habits, and the relationship between the N.S.A. and telecom companies—become so thoroughly unlike those considered by the Supreme Court thirty-four years ago that a precedent like Smith simply doesn’t apply? The answer, unfortunately for the government, is now.”

Leon then detailed the many differences between the two cases. While the pen register in the Smith case collected a small amount of data for a brief period of time, the N.S.A. program “involves the creation and maintenance of a historical database containing five years’ worth of data.” He notes with apparent shock that the duration of the program “could be forever!”