A federal appeals court appears largely unconvinced that the government's once-secret practice of collecting virtually all Americans' phone records violates the Constitution.

A panel of three Republican-nominated judges on the D.C. Circuit Court of Appeals challenged arguments made Tuesday by a conservative activist and civil-liberties groups that the National Security Agency's mass-surveillance program represents a breach of the Fourth Amendment, which guards against unreasonable searches.

That spy program, exposed by Edward Snowden last year, allows the government to collect from telephone companies, such as Verizon and AT&T, the "metadata" records of their customers. Metadata includes the numbers, dates, and duration of calls but not the actual contents of conversations.

The case, Klayman v. Obama, is one of three currently at the appeals-court level that is weighing NSA surveillance. The New York-based 2nd Circuit Court of Appeals held a review in September and appeared more sympathetic at the time to the concerns of privacy advocates. Many observers expect some combination of these cases to reach the Supreme Court as soon as next year, but much of that may depend on whether Congress decides to pass legislation that would reform certain aspects of the NSA's spying apparatus.

While privacy advocates have argued that metadata collection has the potential to be abused and can be extremely intrusive, two of the three judges repeatedly voiced skepticism that the mere collection of those records—which are already maintained by telecom companies—poses any threat to ordinary citizens.

Judge Stephen Williams indicated that violations to privacy were more likely to occur when intelligence officials actually analyze metadata, which he said was "two steps" removed from the collection stage.

Larry Klayman, a conservative lawyer arguing on his own behalf, countered that "just collecting the data is enough to implicate the Fourth Amendment."

Williams pressed Klayman to articulate specifically how the mere collection of troves of unanalyzed data could violate an American's privacy. But Williams remained unsatisfied with Klayman's response.

"You still aren't answering my question," Williams interrupted at one point. "What are the invasions?"

Much of Tuesday's oral argument hinged on whether the NSA's surveillance of phone records was more intrusive than the government's use of so-called pen registers, a decades-old wiretap technology that allows authorities to record numbers called from a tapped phone line. The Supreme Court ruled in a 1979 case, Smith v. Maryland, that installing pen registers did not constitute a search as understood under the Fourth Amendment and thereby did not require a warrant, because those numbers were already recorded by the telephone company.

During one prolonged exchange, Judges Williams and David Sentelle both asked Klayman whether the NSA's bulk collection of phone records was substantively different than the information at issue in theSmith case.

Klayman struggled to answer this question, and instead fell back on sweeping statements warning of the grave dangers of unchecked government surveillance.

"This is the most outrageous abuse of our liberties in history," Klayman said at one point, slightly raising his voice and eliciting some snickers from the audience. "This is what we fought the Revolution for.... If the courts don't step in, we are going to be in the streets again."

Cindy Cohn, an attorney from the Electronic Frontier Foundation, an open-Internet group, also was allowed to argue in support of Klayman's suit. Representing her organization as well as the American Civil Liberties Union, Cohn said that the size of the NSA bulk-collection program made it more intrusive than pen registers.

"A difference in kind can come from a difference in scale," Cohn said. "The aggregation gives a more intrusive look at someone's life."

But Judge Sentelle indicated skepticism that the size of the NSA's metadata dragnet mattered. "Does a thousand times nothing still equal nothing?" he asked.

Justice Department attorney Thomas Byron repeatedly defended the government protocol, noting that it was an intelligence program that helped preserve national security and was subject to heavy oversight. He questioned whether Klayman had standing in this case, noting that he is a Verizon wireless customer and that the Snowden documents only specifically confirm that the government has requested metadata from a Verizon subsidiary known as Verizon Business Network Services. NSA critics have routinely dismissed that argument, and say it further demonstrates how secretive the government continues to be about its intelligence-gathering practices.

The judges also seemed disinclined to consider a separate petition arguing that the government has misinterpreted Section 215 of the post-9/11 Patriot Act, a law that intelligence officials say governs their ability to collect bulk-communications data.

The bulk collection of American phone data was the first of the NSA's sweeping surveillance programs that came to light due to the Snowden leaks last year. It has been subject to more debate in all three branches of government than any other program, and has drawn heavy criticism from the tech industry as well as privacy and civil-liberties groups.

Efforts are underway in Congress to curtail the phone-records program, by keeping the data with telecom companies, from whom the government could request it after obtaining a warrant from the Foreign Intelligence Surveillance Court. But as the judges noted on Tuesday, legislative action could complicate or even render moot its legal opinion.

Though three appeals courts are in the process of weighing the legality of NSA surveillance, Tuesday's hearing is the only one to follow a lower-court ruling that largely condemned the government's spying apparatus. A federal judge, Richard Leon, in December characterized the NSA's metadata acquisition as an "almost-Orwellian technology" that likely represented a Fourth Amendment breach. That lower-court ruling included a temporary injunction blocking the NSA's collection of phone records, but that action was halted pending appeal.

An opinion by the D.C. Appeals Court is not expected until at least early next year. All three judges—Williams, Sentelle, and Janice Rogers Brown—have generally sided with the government in cases involving national security.