This spring, Congress appeared primed for an Edward Snowden-fueled pullback of the nation's spy programs. Then the world fell apart.

The group Islamic State in Iraq and Syria publicly executed two Americans as part of a bloody terror campaign, a commercial airliner was shot down in the Ukraine-Russia conflict, and violence reignited in Gaza between Israel and Hamas.

Now, the summer's horror has placed a new hurdle in the path of those seeking to curb the power of the National Security Agency—and offered spy programs' supporters a new line of attack.

"If people don't get what we're saying about the threats to our homeland now, shame on the Congress," said Republican Sen. Lindsey Graham, one of the Senate's strongest opponents of pulling back the NSA's capabilities.

"When you've got a terrorist that you're monitoring, I want to know who they're calling. I want oversight, I want judicial review. But now is not the time to degrade our capability to pick up an attack before it happens," Graham said. "The goal is to hit them before they hit us."

The renewed saber-rattling in Washington has some privacy groups concerned that lawmakers won't stomach a vote right now on something that could be perceived as reining in the national security apparatus.

"You can't deny that no lawmaker wants to be seen as causing the next terrorist attack. They want to be seen as taking the next steps to keep us more secure," said Amie Stepanovich, executive director of the public interest group Access, which supports NSA reform. "However, I would caution against the 'throwing everything at the wall' approach that has shown no result … and not just turning to surveillance as a crutch."

Hanging in the balance of this debate is legislation from Senate Judiciary Chairman Patrick Leahy. The bill deals with the collection of metadata—phone numbers and call durations and times, but not actual conversations—on virtually all calls within the United States. The practice, revealed as part of the Snowden leaks, created an instant controversy and has been a top target of privacy advocates.

Leahy's bill would halt the bulk metadata collection. Instead, phone companies would retain those records, and intelligence agencies could obtain them only after earning approval from the Foreign Intelligence Surveillance Court.

Leahy's measure has earned backing from privacy groups, tech companies, the administration, and conservative firebrand Sen. Ted Cruz. Even Director of National Intelligence James Clapper, in a letter coauthored by Attorney General Eric Holder, said last week that the bill "preserves essential intelligence-community capabilities."

But Leahy's impressive coalition hasn't been enough—at least thus far—to score a Senate floor vote. And even before the summer elevated Americans' security concerns, the reform package faced major obstacles.

Apart from a jam-packed schedule, policy differences among key lawmakers remain a chief hurdle.

Senate Intelligence Chairwoman Dianne Feinstein is leading a coalition of those who believe the bill goes too far. And her demand that any legislation require phone companies to store metadata for a longer period of time than they already do has for months been a sticking point in the discussions. Privacy groups and tech companies insist any such provision would amount to a "poison pill" that would kill their support for the measure.

On the other side, Sens. Ron Wyden and Mark Udall—standard-bearers for the surveillance-reform movement—have also not embraced the Freedom Act. The Democrats want to strengthen the bill to require warrants for so-called "backdoor" searches of Americans' Internet data, which can be incidentally collected during foreign surveillance. Despite an amendment to that effect passing the House, Wyden and Udall have encountered resistance in the Senate.

Still, most advocates say it's more a matter of when, not if, NSA reform happens. Election-season calendars, they say, always leave some legislation on the cutting-room floor that Congress knows it has to revisit later.

Wyden in particular sees no need to rush an NSA bill this year (despite his ally Udall being caught in a tough reelection campaign), believing that the expiration next summer of Section 215 of the USA Patriot Act—the post-9/11 legal doctrine for the NSA's phone-records collection—could buy him a better hand.

"This is the first time since I've been at Intelligence where the clock [has] favored the reformers," he said. "Historically what happens is people who are sort of for the status quo bump it up against the date when it expires. We all say, 'It's a dangerous world,' which it certainly is—we've seen that again in the last few weeks—and then it just extends what you have."

Regardless of the political climate, NSA critics maintain that the bill would do nothing to jeopardize national security. "I don't see any evidence that collecting millions and millions of phone records on law-abiding Americans is going to in some way affect our ability to deal with that very serious threat," Wyden said.

"We're always going to face threats," Leahy said. "But the biggest one we don't want to face is a threat to our own privacy."

Brendan Sasso contributed to this article.