For the first time since the revelation of the National Security Agency’s vast dragnet of all Americans’ telephone records, a federal court has ruled that such surveillance is “significantly likely” to be unconstitutional.

In a scathing 68-page opinion peppered with exclamations of incredulity, U.S. District Judge Richard Leon found that the seven-year-old phone-data collection program — established under the USA Patriot Act and repeatedly reauthorized by a secret intelligence court — “almost certainly” violates the Fourth Amendment’s prohibition against unreasonable searches.

Reaching into the 18th century, the judge wrote James Madison “would be aghast” at the degree of privacy invasion the data sweep represents.

The ruling by Judge Leon, who was nominated to the bench by President George W. Bush on Sept. 10, 2001, was remarkable for many reasons, but mainly because there were real people sitting in open court challenging the government’s lawyers over the program’s constitutionality.

The plaintiffs, led by Larry Klayman, a conservative legal activist, sued the government after the program came to light. A similar suit filed by the American Civil Liberties Union is in a federal court in New York.

Judge Leon’s opinion took issue with the government’s reliance on a 1979 Supreme Court case, Smith v. Maryland, which upheld the police’s warrantless capture of phone numbers dialed from the home of a robbery suspect on grounds that the suspect had no reasonable expectation of privacy.

But the NSA’s phone-surveillance program is “a far cry” from what the court considered in 1979, Judge Leon wrote. While the circumstances in the Smith case involved a “one-time, targeted request for data regarding an individual suspect in a criminal investigation,” the phone-surveillance program is a “daily, all-encompassing, indiscriminate dump” of data from the phones of people who are not suspects.

Further, he reasoned, the Supreme Court in 1979 could not “have ever imagined how the citizens of 2013 would interact with their phones.”

“Thirty-four years ago, when people wanted to send ‘text messages,’ they wrote letters and attached postage stamps,” Judge Leon wrote. In other words, as technology changes, so does the expectation of privacy — the standard by which a court determines whether a search is reasonable.

Judge Leon recognized the government’s compelling interest in preventing terrorism, but pointed out it “does not cite a single instance” in which data sweeps “actually stopped an imminent attack.”

In order to reach these issues, Judge Leon first ruled plaintiffs had standing — that is, they have the legal capacity to challenge the collection of the phone data and the subsequent searches of that data. That is significant, because prior to the revelation of the phonedata sweep, the Supreme Court had rejected a similar challenge because plaintiffs could not prove that the government had ever collected their personal data. As Verizon customers, Mr. Klayman and the other plaintiffs can now show “strong evidence” of that collection, the judge ruled.

The judge, in granting the plaintiffs’ request for an injunction, ordered the government to stop collecting plaintiffs’ phone data and to destroy any data it had collected, but because of the “significant national security interests at stake,” he stayed his own ruling to allow the government to appeal. The decision applies only to the plaintiffs in this case, and not to the American public at large.

It is an enormous symbolic victory for opponents of the bulk-collection program. For seven years, these constitutional issues have been adjudicated under a cloak of secrecy. That cloak has finally been lifted in a true court of law.