The government has argued that under a 1979 Supreme Court ruling, Smith v. Maryland, no one has an expectation of privacy in the telephone data that phone companies keep as business records. In that ruling, the high court rejected the claim that police need a warrant to obtain such records.
But Leon said that was a "far cry" from the issue in this case. The question, he said, is, "When do present-day circumstances -- the evolutions in the government's surveillance capabilities, citizens' phone habits, and the relationship between the NSA and telecom companies -- become so thoroughly unlike those considered by the Supreme Court 34 years ago that a precedent like Smith simply does not apply? The answer, unfortunately for the government, is now."
He wrote that the court in 1979 couldn't have imagined how people interact with their phones nowadays, citing the explosion of cellphones. In addition, he said, the Smith case involved a search of just a few days, while "there is the very real prospect that the (NSA) program will go on for as long as America is combating terrorism, which realistically could be forever!"
Leon added: "The almost-Orwellian technology that enables the government to store and analyze the phone metadata of every telephone user in the United States is unlike anything that could have been conceived of in 1979."
The judge also mocked the government's contention that it would be burdensome to comply with any court order that requires the NSA to remove the plaintiffs from its database.
"Of course, the public has no interest in saving the government from the burdens of complying with the Constitution!" he wrote. As for the government's complaint that other successful requests "could ultimately have a degrading effect on the utility of the program," he said, "I will leave it to other judges to decide how to handle any future litigation in their courts."
Sen. Mark Udall, a Colorado Democrat and member of the Intelligence Committee, said Leon's ruling "underscores what I have argued for years: The bulk collection of Americans' phone records conflicts with Americans' privacy rights under the U.S. Constitution and has failed to make us safer."
Stephen Vladeck, a national security law expert at the American University law school, said Leon is the first judge to say he has serious constitutional concerns about the program.
"This is the opening salvo in a very long story, but it's important symbolically in dispelling the invincibility of the metadata program," he added.
Vladeck said 15 judges on the Foreign Intelligence Surveillance Court have examined Section 215 of the Patriot Act, the provision of law under which the data collection takes place, without finding constitutional problems. "There's a disconnect between the 15 judges on the FISA court who seem to think it's a no-brainer that Section 215 is constitutional, and Judge Leon, who seems to think otherwise."
Vladeck said there is a long road of court tests ahead for both sides in this dispute and that a higher court could ultimately avoid ruling on the big constitutional issue identified by Leon. "There are five or six different issues in these cases," Vladeck said.
Robert F. Turner, a professor at the University of Virginia's Center for National Security Law, said searching the databases involved in the National Security Agency case is similar to searching motor vehicle records or FBI fingerprint files.
The judge's decision is highly likely to be reversed on appeal, Turner said.
He said the collection of telephone metadata -- the issue in Monday's ruling -- has already been addressed and resolved by the Supreme Court. Turner said law enforcement officials routinely obtain telephone bills that include the numbers dialed without the use of a warrant.
"The odds that an American will have their phone metadata examined by law enforcement officials are about a thousand times greater than by the National Security Agency," Turner said.
Jameel Jaffer, deputy legal director of the American Civil Liberties Union, which has a similar challenge pending in federal court in New York, called Leon's ruling "a strongly worded and carefully reasoned decision that ultimately concludes, absolutely correctly, that the NSA's call-tracking program can't be squared with the Constitution."Associated Press writers Mark Sherman, Pete Yost, Nedra Pickler and Kimberly Dozier in Washington and Bradley Brooks in Brazil, contributed to this story.