While Congress mulls how to curtail the NSA's collection of Americans' telephone records, impatient civil liberties groups are looking to legal challenges already underway in the courts to limit government surveillance powers.

Three appeals courts are hearing lawsuits against the bulk phone records program, creating the potential for an eventual Supreme Court review. Judges in lower courts, meanwhile, are grappling with the admissibility of evidence gained through the NSA's warrantless surveillance.

Advocates say the flurry of activity, which follows revelations last year by former NSA contractor Edward Snowden of once-secret intelligence programs, show how a post-9/11 surveillance debate once primarily hashed out among lawmakers in secret is being increasingly aired in open court — not only in New York and Washington but in places like Idaho and Colorado.

"The thing that is different about the debate right now is that the courts are much more of a factor in it," said Jameel Jaffer, deputy legal director at the American Civil Liberties Union. Before the Snowden disclosures, he said, courts were generally relegated to the sidelines of the discussion. Now, judges are poised to make major decisions on at least some of the matters in coming months.

Though it's unclear whether the Supreme Court will weigh in, the cases are proceeding at a time when the justices appear increasingly comfortable with digital privacy matters — including GPS tracking of cars and police searches of cellphones.

The cases "come at a critical turning point for the Supreme Court when it comes to expectations of privacy and digital information," said American University law professor Stephen Vladeck.

Revelations that the government was collecting phone records of millions of Americans who were not suspected of crimes forced a rethinking of the practice, and President Barack Obama has called for it to end.

Since then, the House has passed legislation that civil libertarians say did not go far enough. In the Senate, Vermont Democrat Patrick Leahy, the Judiciary Committee chairman, is seeking a vote on a stricter measure to ban bulk collection, and it has bipartisan backing and support from the White House.

As Congress considers the matter, the federal judiciary has produced divided opinions.

The New York-based 2nd U.S. Circuit Court of Appeals recently heard arguments in an appeal of a judge's opinion that had upheld the program's legality. The D.C. appeals court hears arguments next week after a judge there found that the program is probably unconstitutional. Anna Smith, a nurse in Idaho who contends the program is unconstitutional and she is at risk of having her records collected, will soon have her appeal heard by the appeals court in the 9th Circuit. And a Somali cab driver convicted in California of funneling money to a terror group is challenging a phone records program the government said was vital for his prosecution.

Any court opinion before Congress takes action could influence the lawmakers' debate. Congress could also act first, but even if it clears up questions about the government's statutory authority to collect phone records, courts might still confront constitutional questions.

Besides those cases, multiple defendants who have been notified in the past year that the Justice Department used NSA-derived evidence against them in their prosecutions are now challenging the government. At issue is a provision of the Foreign Intelligence Surveillance Act known as Section 702. It permits the NSA to sweep domestic communications of any American in contact with a terror suspect, even if those contacts aren't related to terrorism. These defendants' challenges would have been impossible until last year, when the Justice Department began notifying them that it had gathered evidence through warrantless surveillance.

A judge in Oregon rejected an effort by Mohamed Mohamud, a Somali-American convicted of plotting to detonate a bomb, to suppress such evidence and sentenced him to 30 years in prison, though that issue will likely resurface on appeal. A similar defense request is pending in the Colorado case of Jamshid Muhtorov, who is charged with trying to travel overseas to fight for an Uzbek terrorist group and wants information about the government's surveillance methods so he can challenge their legality. In Chicago, a man charged with trying to ignite a bomb outside a bar is scheduled for trial next year after fighting unsuccessfully to see secret intelligence-court records.

In New York, Albanian citizen Agron Hasbajrami, who admitted trying to go to Pakistan to join a jihadi group, recently withdrew his guilty plea following the government's notification of how it obtained evidence in his case.

Congressional supporters of limiting surveillance see an urgent need for action, and say changes are better addressed through legislation than litigation.

A critical deadline is June 1, 2015, when the section of law authorizing the bulk records collection is set to expire. If no action is taken before then, that could lessen the chances of a Supreme Court review. Congress may also wind up acting first, which could resolve some of the outstanding statutory issues.

The ACLU's Jaffer said he hopes that Congress will overhaul the program but that courts also have a natural role to play.

"To the extent that Congress is authorizing mass surveillance of Americans' telephone calls, the Constitution has something to say about that — and only the courts are in a position to enforce the Constitution," he said.

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