A federal Appeals Court this week will review the government's controversial domestic spying program that collects the phone records of virtually all Americans, a once-secret practice that was publicly exposed last year by Edward Snowden.

A three-judge panel of the D.C. Circuit Court of Appeals will hear arguments Tuesday considering the constitutionality of the National Security Agency's bulk collection of phone "metadata"—the numbers, dates, and duration of calls but not the actual content of conversations. The case is poised to have far-reaching implications for digital privacy and could appear before the Supreme Court as soon as next year.

At issue is whether the government's gathering of American phone records is a violation of the Fourth Amendment's protection against unreasonable searches. The government has said that the practice, authorized under Section 215 of the post-9/11 USA Patriot Act, does not amount to a legitimate invasion of privacy and is necessary to detect national-security threats—although some intelligence officials have in recent months indicated that the program in its current form may be unnecessary.

Privacy advocates, however, argue that phone metadata can be extremely revealing, and that the government's collect-it-all approach is unduly intrusive without accomplishing its stated purpose of preventing terrorist attacks.

The bulk collection of American phone data was the first spying program revealed by leaks from Snowden, a former NSA contractor, last year. It has been the most hotly debated program unearthed by his trove of top-secret documents, prompting considerable review of the technique across all three branches of government.

Yet despite a promise from President Obama and efforts in Congress to rein in the NSA, few reforms have been enacted, even despite findings by a presidential review board and the government's independent privacy watchdog that concluded that bulk phone surveillance was illegal and yields little to no national security benefit.

In response to inaction elsewhere, anti-surveillance activists believe the courts may ultimately provide the best way forward to reforming the government's surveillance state.

The lawsuit, brought by conservative activist Larry Klayman in the wake of the Snowden revelations last year, represents one of several challenges to the government's sweeping spying powers currently winding their way through the courts, movement made more notable by continued inaction in Congress or the executive branch to reform NSA spying.

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.

In September, the Court of Appeals for the 2nd Circuit heard another suit, ACLU v. Clapper,challenging the NSA's phone spying on similar constitutional grounds. Unlike Klayman's suit, the lower court in that case defended NSA spying as a necessary and effective response to terrorist threats such as al-Qaida.

A third case challenging the program, Smith v. Obama, is set to undergo review in December by the Court of Appeals for the 9th Circuit. The San Francisco-based court is commonly regarded as one of the most left-leaning and sympathetic to the concerns of the tech industry. That court also recently reviewed whether the FBI can compel companies to hand over communications data or financial records of users for national-security investigations in conjunction with a gag order.

The flurry of judicial action suddenly lurching forward contrasts with slow-burning efforts in Congress to curtail NSA spying, where negotiations have repeatedly been slowed despite efforts by members in both chambers to pass a bill this year. President Obama in January pledged to reform the government's surveillance programs, but said he had to wait until lawmakers put a suitable bill on his desk to do so.

This fall, Senate Judiciary Committee Chairman Patrick Leahy introduced a bill, the USA Freedom Act, that would essentially end the government's bulk collection of phone metadata. Under the measure, phone companies would instead keep those records, which intelligence agencies could request only after being given approval from the Foreign Intelligence Surveillance Court, except in emergency cases. The Freedom Act would also install several additional privacy and transparency measures on NSA spying, including a more precise definition for what is allowed to be considered a surveillance target.

The Freedom Act has earned support from the tech industry, most privacy and civil-liberties groups, and the White House. It even won an unlikely endorsement from Director of National Intelligence James Clapper. Despite the backing, however, it remains unclear if the bill will earn a lame-duck vote this year, as both national security hawks and privacy hard-liners, including Sen. Ron Wyden, have voiced concerns.

But any action in Congress could dictate how courts ultimately go forward with their reviews of NSA spying. A sudden dismantling of the Patriot Act, or a significant change, could render judicial reviews essentially moot, according to legal observers.

"It's a very likely scenario that the Supreme Court will review this," said Patrick Toomey, a lawyer with the American Civil Liberties Union. "But it depends on what Congress does."

The Electronic Frontier Foundation, representing itself and the ACLU, will also argue before the court Tuesday on behalf of Klayman. Additionally, the Center for National Securities Studies will present an argument challenging the government's statutory interpretation of the Patriot Act.

A decision by the D.C. Appeals Court is not expected until at least early next year.