The House overwhelmingly approved an amendment Thursday meant to block the National Security Agency from performing warrantless searches on Americans' communications, rejecting one of the most controversial forms of NSA surveillance revealed by the leaks of Edward Snowden.

The 293-123 vote on an amendment to the annual defense appropriations bill was a victory for civil libertarians on the heels of a gutted NSA reform measure the House approved in May. The amendment still faces an uncertain journey through Congress before it can become law.

"I think people are waking up to what's been going on," said the amendment's sponsor, Rep. Thomas Massie (R-Ky.). "Whether you're Republican or Democrat -- because, if you noticed, a majority of Republicans voted for this, as well as a majority of Democrats."

The NSA performs so-called back door searches on the content of Americans' communications without a warrant when they have been in contact with targeted foreigners. Given the vastness of the NSA's target database, and the irrelevance of international boundaries in the Internet age, privacy advocates say they worry an expanding number of Americans' emails and phone calls are being swept up. The House amendment specifically prohibits the NSA from using information identifying U.S. citizens to search communications data it collects under the Foreign Intelligence Surveillance Act.

NSA critics had warned about such searches for years. But the leaks of former NSA contractor Snowden, as reported in The Guardian, definitively revealed the spy agency's tactics. The NSA claims legal authority to perform the searches.

The amendment was sponsored by Massie, along with Rep. Zoe Lofgren (D-Calif.) and a host of co-sponsors of the earlier attempt at NSA reform, called the USA Freedom Act. The original co-sponsors of that bill included Rep. James Sensenbrenner (R-Wis.), Rep. John Conyers (D-Mich.), Rep. Jerrold Nadler (D-N.Y.) and Rep. Justin Amash (R-Mich.).

Supporters of that measure had surrendered to the Obama administration in dropping the prohibition on back door searches. But the defense appropriations bill gave them another chance.

"After the passage of the USA Freedom Act, this amendment is the logical next step to prevent improper surveillance," Nadler said in a statement before Thursday's vote. "I will continue to work to improve our nation’s privacy laws and to ensure that this Administration, and all those that follow it, respect the constitutional rights of all Americans."

The amendment also aims to block the NSA and CIA from forcing software and hardware providers to insert back doors into their products to allow the government agencies easy access to customer communications. The measure's backers include the American Civil Liberties Union and Google.

Massie pronounced himself "pleasantly surprised" with his amendment's passage. But he acknowledged it still faces a long road to passage.

"That's going to be the trick," Massie said. "It would take somebody to support it on the Senate side as well, and in conference," where the House and Senate reconcile companion bills.

Massie said he faced hurdles from House leaders -- particularly those in charge of the House Intelligence and Judiciary committees -- in winning approval for the amendment. "The leadership was not in favor of it," he said. "The whip's description of the bill, which you could pick up in the cloakroom, was very unflattering and misleading in my opinion.

"It was an amazing struggle to get it ruled in order," he said, referring to parliamentary rules around appropriations bills. He noted that a similar provision was stripped out of the "watered down" USA Freedom Act that passed last month.

But in the end, Massie said, co-sponsors from both parties were critical in getting the amendment approved.

The House passage of the amendment may create momentum for similar reform efforts in the Senate, where Oregon Democrat Ron Wyden has been fighting for years to expose and end the practice of back door searches.

Wyden warned in a recent Los Angeles Times opinion piece with Sens. Mark Udall (D-Colo.) and Rand Paul (R-Ky.) that "intelligence agencies are using a loophole in the law to read some Americans' emails without ever getting a warrant."

The senators wrote that the debate over NSA reforms "is likely to continue for at least the next few years as Americans continue to learn about the scale of ongoing government surveillance activities."