At 3:59 p.m. EDT on Sunday, the National Security Agency and telecommunications companies will begin mothballing a once-secret system that collected Americans' bulk telephone records, shutting down computers and sealing off warehouses of digital data.
If the U.S. Congress fails to act, key provisions of the USA Patriot Act will lapse in a watershed moment in the post-Sept. 11, 2001, era. Intrusive government powers, created and wielded in the name of preventing another mass-casualty terrorist attack, would be at least partly scaled back, proponents and critics of the surveillance say.
The Federal Bureau of Investigation, for instance, will no longer be able to employ 'roving wiretaps' aimed at terrorism suspects who use multiple disposable cell phones, and will have more difficulty seizing such suspects' and their associates' personal and business records.
"We're past the immediate aftermath of the 9/11 attack. And we can look at these issues more calmly," said Peter Swire, who served on a review panel appointed by President Barack Obama after former contractor Edward Snowden's 2013 revelations of vast NSA surveillance.
With the clock ticking, a coalition of Senate Republican libertarians and security hawks has blocked action on new legislation known as the USA Freedom Act that would reform the bulk telephone data program but not kill it.
Libertarians want the program ended altogether, while the hawks argue it should be maintained as it is now.
Senator Rand Paul, who last week used a long speech to target the extension, on Saturday said he would block it again.
"Tomorrow, I will force the expiration of the NSA illegal spy program," the Kentucky Republican said in a statement.
Currently, telecom providers are legally required to send phone records to the government. The USA Freedom Act would require private firms to hold the data, which the NSA could search with court authorization.
The U.S. Senate is scheduled to hold a special session to consider the legislation at 4 p.m. (2000 GMT) on Sunday, just as security officials say they have to begin shutting the NSA program down to meet a midnight (0400 GMT) deadline. The USA Freedom Act already has passed the House of Representatives and has Obama's strong support.
It is unclear if supporters of the Freedom Act can get the 60 votes needed in the Senate to move forward. A previous attempt on May 23 fell short, 57-42, but the bill's backers have been pushing hard to win over three more senators.
How badly U.S. counter-terrorism efforts would be disrupted by even a temporary suspension of the telephone data collection and other legal authorities is disputed.
The Obama administration is issuing increasingly dire warnings, sometimes citing Islamic State's calls on its supporters to conduct attacks wherever they live.
"The intelligence community will lose important capabilities," Director of National Intelligence James Clapper said in a statement. "Prompt passage of the USA Freedom Act by the Senate is the best way to minimize any possible disruption of our ability to protect the American people."
But many experts and civil liberties advocates say that U.S. intelligence and law enforcement authorities have other powerful, and less objectionable, tools to investigate and prosecute militant plots. Those include court orders, subpoenas and other forms of electronic surveillance.
"The government still has expansive ... law enforcement tools that will remain in place," said Anthony Romero, executive director of the American Civil Liberties Union.
Groups as diverse as the left-leaning ACLU and the conservative Tea Party Patriots argue the telephone data program is unconstitutionally broad, targeting the communications of millions of innocent Americans.
This month, a federal appeals court ruled the program was illegal, going beyond what the Patriot Act authorized. The court declined to halt the program, saying it would give Congress a chance to act.
"COLLECTING THE DOTS"
The bulk telephone records program, first exposed to journalists by Snowden, collects metadata, which is data that provides information about other data, about U.S. citizens' phone calls. This includes the number dialed and time and length of the call but not its content.
Senior U.S. officials have said the surveillance fills a critical gap, determining whether a militant overseas is communicating with someone inside the United States.
Swire, a privacy and cyber security expert and a professor of law and ethics at the Georgia Institute of Technology, said the panel he served on "looked at the classified file and concluded that telephone metadata had not been essential to preventing any attack."
Current and former U.S. intelligence officials say that is the wrong yardstick to use.
"Very seldom do we have one (single) piece of information" that prevents a terrorist attack, said Richard Schaeffer, a former top NSA official. "It is literally collecting the dots" and piecing them together, he said.
Current U.S. officials seem even more chagrined about losing other powers that could expire at midnight Sunday. Those, they argue, were never controversial but have been caught up in the backlash against government intrusion.
One, known as the "lone wolf" provision, allows the FBI to seek a wiretap on an individual suspected of terrorist activity, even if the person cannot be linked to a specific group. Law enforcement officials have never used that power.
But, a senior U.S. official said, "this is not a tool that we want to see go away."
The provision giving the FBI expanded powers to seek a terrorism suspect's records such as hotel stays and car rentals is used about 200 times per year, officials said.
The roving wiretap provision, in which a court authorizes tapping an individual's communications regardless of the device used, rather than a specific phone number, has been used most commonly in law enforcement rather than terrorism cases, said Stewart Baker, a former top Homeland Security official.
"But it's not an authority we would want to lose," he said.