The U.S. Senate is set to convene in a rare Sunday session in a last-ditch attempt to pass legislation to allow U.S. spy agencies to continue to sweep up information on Americans' telephone calls and other business records.
Failure to pass such legislation would mean that key provisions of the USA Patriot Act would expire and, facing a midnight (0400 GMT Monday) deadline, the National Security Agency would have to shut off a vast surveillance system.
The Patriot Act was signed into law by Republican President George W. Bush after the Sept. 11, 2001 attacks on the United States and parts of it have been renewed under Democratic President Barack Obama.
But with the clock ticking on some sections of the act, efforts to renew them have stalled in the Senate, which also has failed to advance a compromise bill known as the USA Freedom Act that would reform the telephone data program.
Libertarians want the program ended altogether, while security hawks want it extended, unchanged.
The Senate is scheduled to hold a special session to consider the legislation at 4 p.m. EDT (2000 GMT) on Sunday, just as security officials say they have to begin shutting down the NSA program to meet the midnight deadline.
The Freedom Act, which ends the spy agencies' bulk collection of domestic telephone "metadata" and replaces it with a more targeted system, has already passed the House of Representatives by an overwhelming margin and has Obama's strong support.
Senator Rand Paul, a libertarian campaigning to become the Republican candidate in next year's presidential election, has vowed to block any extension of the program, calling it government intrusion on privacy rights.
It is unclear if supporters of the Freedom Act can get the 60 votes needed to move it forward in the 100-member Senate.
A previous attempt on May 23 fell three votes short and the bill's backers have been pushing hard to sway three more senators.
"I do believe we have the votes," Republican Senator Mike Lee told CNN's "State of the Union" program on Sunday. "It'll happen either tonight or it'll happen on Wednesday or sometime in between then."
CIA Director John Brennan, appearing on CBS's "Face the Nation" program, said data collection was "important to American lives" and that being without them could mean missing warning of a big attack on the United States.
Under the USA Freedom Act, the telephone records would be held by telecommunications companies, not the government, and the NSA would have to query them to look for specific data.
The existence of the NSA program was revealed by former contractor Edward Snowden in 2013, prompting calls for reform.