US congressmen have forced a vote that could cut funding for the National Security Authority (NSA) and limit the scope of its vast surveillance activities.
The vote, which could come as early as Wednesday in the House of Representatives, will be the first held on limiting NSA spying since Edward Snowden leaked documents detailing the agency's extensive secret surveillance programme known as Prism.
Republican congressman Justin Amash tabled an amendment to a wider defence bill, which "ends authority for the blanket collection of records under the Patriot Act", and limits surveillance to individuals under investigation.
House opponents to Prism have tabled a second amendment to cut back the NSA's funding.
The US president, Barack Obama, who has spoken out in defence of the programme and said the widespread spying has prevented more than 50 "terrorist plots", sent the NSA chief, General Keith Alexander, to Capitol Hill to urge both Republican and Democratic politicians to oppose the move to restrict the agency's activities.
"We oppose the current effort in the House to hastily dismantle one of our Intelligence Community's counterterrorism tools," White House spokesman Jay Carney said in a late-night statement. "This blunt approach is not the product of an informed, open or deliberative process."
Amash tweeted on Tuesday: "Do Members of Congress want to go on record supporting unconstitutional, blanket collection of all Americans' phone records? We'll find out." Later, he tweeted: "US gov't has come out in full force against you, the American people. I will always stand with you & the Constituion I swore to defend."
Carney said Obama was still open to discussing the balance between security and privacy with Congress and Americans - but that he wants an approach that properly weighs what intelligence tools are needed to keep Americans safe.