At midnight Sunday, barring any last-minute deal in Congress, a key section of the US Patriot Act which has been used as a legal basis for much of the vast surveillance carried out by the NSA will expire or "sunset."
This would likely shut down most "bulk collection" efforts of US intelligence and law enforcement which have sparked outrage since revelations on the scope of the programs from former NSA contractor Edward Snowden.
A senior administration official said the switch would be turned off for the bulk collection servers at 4:00 pm (2000 GMT) and any collection after midnight would be deemed illegal, without congressional authorization.
The deadline has led to a frenzy of activity in Congress to keep programs intact for national security investigations, but the outcome is far from clear.
The House of Representatives earlier this month passed the USA Freedom Act which would rein in NSA authority by ending bulk collection, and improve transparency at a secret court which supervises the program.
But the Senate blocked a vote on the bill, and failed to muster support for a short-term extension of the law, Section 215 of the Patriot Act.
Some senators have offered compromise reform measures, but it remains unclear if those bills can garner enough support to win passage in the Senate or prompt the House to reconsider its own measure.
Harley Geiger at the Center for Democracy & Technology, which has led a campaign for the USA Freedom Act, said the compromise bills proposed are far weaker in reining in the surveillance.
Geiger said that if the law expires, even for just one day, it may change the dynamic of the vote process because lawmakers would be reinstating surveillance authority and not simply extending existing programs.
"Civil liberties groups are united in opposing anything weaker than the USA Freedom act," he said.
Members of the House have also warned that they may not accept a different measure from the Senate or extend the existing law.
"If the Senate chooses to allow these authorities to expire, they should do so knowing that sunset may be permanent," said a statement from leaders of the House Judiciary Committee.
Adding to the confusion is a US appeals court ruling which said the government went beyond the intent of Congress with bulk collection and that the program was illegal. This means lawmakers must affirm they want a sweeping surveillance effort which has been fiercely criticized at home and abroad.
Hailing the sunset
Some civil liberties activists welcome the possibility of a "sunset," saying it is better than a weak reform.
"The Patriot Act, rushed through Congress in the wake of a national crisis, included sunset provisions for a reason: The extraordinary new powers created by the law were to be re-examined and allowed to expire if abusive or ineffective. Both of those criteria have been met," said David Segal of the activist group Demand Progress, on behalf of a coalition including Tea Party Nation and the Council on American-Islamic Relations.
Geiger said an expiration would be a mixed outcome and that a better result would be passage of the USA Freedom Act, even if the reforms are relatively modest.
"If we have a sunset, surveillance authority becomes much more narrow," he told AFP.
"But the downside is that a sunset will cause the intelligence agencies to freak out and security hawks in Congress to claim this is a national security crisis."
A last-minute deal?
The White House is urging lawmakers to step up with an agreement before the expiration, to preserve the ability to keep key national security efforts in operation.
"The administration has been in touch with senators over the last week to urge them to do the one thing that will eliminate unnecessary risk to our national security, and that is to pass the USA Freedom Act," White House spokesman Josh Earnest said Friday.
"I haven't heard a rational explanation for what exactly is going on in the United States Senate right now. There is no good explanation for it."
Jameel Jaffer of the American Civil Liberties Union cautioned against "scaremongering" over the sunset.
"The government has many other tools that allow it to collect the same kinds of things that it can collect under Section 215," Jaffer said on the Just Security blog.
Expiration "would undoubtedly be a significant political loss for the intelligence community," he said. "But there's no support for the argument that the sunset of Section 215 would compromise national security."