(AP Photo/Patrick Semansky, File)

A federal judge has called into question a 1979 Supreme Court ruling that is the legal foundation of the National Security Agency's vast phone records program. If upheld on appeal, the judge's reasoning could force the spy agency to reconsider other domestic spying programs that involve warrantless collection of "metadata" about Americans' communication.

The ruling was a challenge to the NSA's phone record program, which involves scooping up records of every American's phone calls. On Monday, in response to a lawsuit from Larry Klayman and Charles Strange, U.S. District Court for the District of Columbia Judge Richard Leon granted a preliminary injunction halting the program from collecting information about the plaintiffs. The ruling won't take effect until the government has a chance to appeal it. This is the first significant legal setback for the phone metadata program revealed in documents leaked by former NSA contractor Edward Snowden.

The most significant part of the ruling may be Leon's parsing of the Smith v. Maryland, a 1979 Supreme Court ruling that is used by a wide variety of law enforcement agencies to justify warrantless surveillance of Americans. The case involved a Baltimore robbery suspect whose phone-call records were collected by the police without a warrant.

The Supreme Court ruled that phone records were not entitled to Fourth Amendment protections. The government has since argued that this applies to other forms of "non-content" communication information -- including the case of the NSA phone metadata program. But many privacy advocates have argued that this broad interpretation doesn't account for how surveillance capabilities have changed since 1979.

On Monday, Leon sided with privacy supporters. In the ruling he specifically argues "the question before me is not the same question that the Supreme Court confronted in Smith," saying that the issue of a pen register is a "far cry" from the issue at hand. Instead, he says the question about the metadata program is better described as:

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 thirty-four years ago that a precedent like Smith simply does not apply?

And just as importantly, he says, "the answer, unfortunately for the government, is now."