WASHINGTON — A federal judge ruled Monday that the National Security Agency’s daily collection of Americans’ phone records is almost certainly unconstitutional.

U.S. District Judge Richard Leon found that a lawsuit by Larry Klayman, a conservative legal activist, has “demonstrated a substantial likelihood of success” on the basis of Fourth Amendment privacy protections against unreasonable searches.

Leon granted the request for an injunction that blocks the collection of phone data for Klayman and a co-plaintiff and ordered the government to destroy any of their records that have been gathered. But the judge stayed action on his ruling pending a government appeal, in recognition of the “significant national security interests at stake in this case and the novelty of the constitutional issues,” Leon wrote in a 68-page opinion.

“I cannot imagine a more ‘indiscriminate’ and ‘arbitrary invasion’ than this systematic and high-tech collection and retention of personal data on virtually every single citizen for purposes of querying and analyzing it without prior judicial approval,” said Leon, a judge on the U.S. District Court for the District of Columbia. “Surely, such a program infringes on ‘that degree of privacy’ that the founders enshrined in the Fourth Amendment.”

The strongly worded decision stands in contrast to the secret deliberations of 15 judges on the nation’s surveillance court, which hears only the government’s side of cases and since 2006 has held in a series of classified rulings that the program is lawful.

A Justice Department spokesman, Andrew Ames, said Monday that the government was reviewing Leon’s decision. “We believe the program is constitutional as previous judges have found,” he said.

The challenge to the NSA’s once-classified collection of phone records is one of a series of cases filed in federal court since the program’s existence was revealed in June by former NSA contractor Edward Snowden.

Snowden praised the ruling in a statement made to journalist Glenn Greenwald, who received NSA documents from Snowden and first reported on the program’s existence.

“I acted on my belief that the NSA’s mass surveillance programs would not withstand a constitutional challenge, and that the American public deserved a chance to see these issues determined by open courts,” said Snowden, who has received temporary asylum in Russia, where he is seeking to avoid U.S. prosecution under the Espionage Act for leaking NSA documents. “Today, a secret program authorized by a secret court was, when exposed to the light of day, found to violate Americans’ rights. It is the first of many.”

The ruling also comes as Congress is debating whether to end the NSA’s “bulk” collection of phone data or endorse it in statute. The White House, U.S. officials say, supports maintaining the program.

“It will be very difficult for the administration to argue that the NSA’s call-tracking program should continue when a federal judge has found it to be unconstitutional,” said Jameel Jaffer, deputy legal director of the American Civil Liberties Union, which has also sued the government over the program’s constitutionality.

But George Washington University law professor Orin Kerr said, “It gives opponents of the NSA program more fuel to add to the fire, but its legal impact is quite limited because the case now just goes to the court of appeals.”

Both of Maine’s U.S. senators – Republican Susan Collins and independent Angus King – serve on the Senate Intelligence Committee responsible for overseeing the NSA and the nation’s other spy agencies.

King said Monday night that he “welcomed the court’s engagement on the issue” but that he had not read Leon’s complete ruling yet.

While not necessarily criticizing collection of the metadata, King has said he is concerned about how it might be used and about allowing the government to hold onto the information. He also sponsored an amendment to an intelligence bill that required the intelligence agencies to report to overseers whenever they queried the metadata.

Ultimately, however, Congress must try to strike a balance between the constitutional requirements to “provide for the common defense” while protecting the Fourth Amendment’s privacy protections, he said.

“I welcome the scrutiny,” King said. “I look forward to participating in the Congress about how to strike the right balance. But ultimately the courts are going to determine whether or not we did it adequately.”

Collins could not be reached for comment Monday evening and her office did not immediately respond to requests for comment.

In a June speech to the World Affairs Council of Maine, Collins defended the telephone data collection program and repeated statements from the NSA’s director, Gen. Keith Alexander, that the surveillance system had detected and thwarted “dozens” of terrorist plots.

“As Congress and the American public learn more about this program, however, there remain questions that must be asked and issues we must address,” Collins said at the time, according to her prepared remarks. “But in doing so, we should not assume a trade-off between liberty and security. Security ensures our freedom.”

The government’s legal justification for the call-tracking program is based on a 1979 case, Smith v. Maryland, which involved the surveillance of one criminal suspect over a two-day period. In that case, the Supreme Court said that Americans have no expectation of privacy in the telephone metadata that companies hold as business records, and that therefore a warrant is not required to obtain such information.

A succession of judges on the Foreign Intelligence Surveillance Court have adopted the government’s argument based on that ruling.

But Leon said the question the Supreme Court confronted in 1979 is not the same as the one he was faced with.

“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 NSA and telecom companies – become so thoroughly unlike those considered by the Supreme Court 34 years ago that a precedent like Smith does not apply?” he wrote. “The answer, unfortunately for the government, is now.”

Kerr said Leon is wrong to suggest that Smith no longer applies. That decision, he said, draws a clear distinction between the collection of data on numbers dialed and on call content. The metadata information the government is gathering today, Kerr said, is the same type of information the court said law enforcement could collect more than 30 years ago.

“The opinion is more valid now than it was,” Kerr said, adding that “it’s up to the Supreme Court to overturn its decision, not trial judges.”

Klayman, the founder of the public interest group Freedom Watch, called Leon’s ruling “courageous.”

“This is a warning to both parties that they’d better start observing the rule of law and protecting the American people or there will be severe consequences,” he said.

Leon rejected the government’s argument that Klayman and a co-plaintiff – the father of an NSA cryptologist killed in Afghanistan in 2011 – lacked standing to bring the suit, because they were customers of Verizon Wireless, which has not been publicly revealed as taking part in the program.

“The government,” he said, says it has created a “comprehensive” database – “in which case, the NSA must have collected metadata from Verizon Wireless, the single largest wireless carrier in the United States.”

Yet, at the same time, he wrote, the government asserts that the plaintiffs lack standing “based on the theoretical possibility” that the NSA has not collected Verizon’s records. “Candor of this type defies common sense and does not exactly inspire confidence!” he wrote.

To draw an analogy using a Beatles rock band reference, he wrote that omitting Verizon Wireless, AT&T or Sprint “would be like omitting John, Paul and George” and building a “Ringo-only database.”

Also Monday, Verizon Communications – the parent company of Verizon Wireless – filed a motion to be dismissed as a defendant in the case, based in part on a certification also filed Monday by the deputy attorney general that the company has immunity against lawsuits since it was compelled by a court order to comply with the government’s request for data. That immunity was authorized under a 2008 law known as the FISA Amendments Act.

Press Herald Washington Bureau Chief Kevin Miller contributed to this report.

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