The Senate entered a debate period late Sunday that will push beyond the midnight deadline, effectively ending the National Security Agency's bulk data collection program since it was first approved in 2006. That's when the government first used the post-9/11 passed Patriot Act as the legal basis for the information collecting program.
The Senate adjourned Sunday night without taking up any further votes, but is expected to move on final passage of a compromise bill called the USA Freedom Act sometime this week.
Sen. Rand Paul, R-Kentucky, took to the Senate floor after opposing the procedural vote to continue lambasting the NSA's domestic surveillance programs, pledging to offer amendments to the House-passed USA Freedom Act in a bid for further reforms. Paul had pledged Saturday to "force the expiration of the NSA illegal spy program."
He acknowledged Sunday that "the bill will ultimately pass," but appeared to reassure his supporters, some of whom packed the Senate gallery.
"The government after this bill passes will no longer collect your phone records," Paul said.
Counterterrorism officials will lose not just the bulk data collection program but also the ability to obtain roving wiretaps to listen in on potential terror suspects, even if they change phones.
Law enforcement officials, though, will be allowed to continue to use roving wiretaps and to collect pinpointed data telecommunications companies and other businesses for ongoing investigations.
Those authorities will likely be restored as early as Wednesday when Republican leadership aides expect a final vote on the compromise bill -- the USA Freedom Act -- which overwhelmingly passed the House two weeks earlier.
But even so, Paul claimed a symbolic victory as he blocked Senate Majority Leader Mitch McConnell from reauthorizing even just the less controversial expiring provisions of the Patriot Act for two weeks.
The Senate then voted 77-17 on Sunday night to cue up a vote on the USA Freedom Act after top Republicans staunchly opposed to changes to that program, including McConnell, reversed course.
Just a week earlier, that same procedural motion failed by three votes.
House Speaker John Boehner on Sunday urged his Senate counterparts to move forward on that House-passed bill, which renews three of the Patriot Act provisions expiring at midnight and would overhaul the controversial bulk telephone collection program, instead requiring a specific, targeted warrant to obtain any call records from telecommunications companies.
If there are any changes to the Freedom Act it would have to go back to the House of Representatives for consideration.
Though McConnell's preference was to renew the Patriot Act as is, he realized that passing a House-passed compromise measure -- the USA Freedom Act -- would be the only option to preserve the expiring Patriot Act provisions.
"It's not ideal but, along with votes on some modest amendments that attempt to ensure the program can actually work as promised, it's now the only realistic way forward," McConnell said Sunday.
McConnell's leadership team the staunchest opponents of NSA reform, including Senate intelligence committee chairman Richard Burr and Sen. John McCain, joined McConnell in voting yes.
"Compromises may have to be made," McCain told reporters on Sunday before heading into the closed strategy session.
McCain and others piled onto Paul earlier on Sunday, noting that his efforts to block the Patriot Act reauthorization are tied to his presidential ambitions.
"I think he obviously has a higher priority for his fundraising and political ambitions than for the security of the nation,' McCain, the chairman of the Armed Services Committee, said on Sunday.
But Paul hit back at his critics during his time on the floor Sunday evening.
"Some of them, I think, secretly want there to be an attack so they can blame it on me," he said.
Heading into a closed-door meeting, Republican Sen. Mark Kirk of Illinois told reporters that he would support the House-passed bill, which he had opposed just last week.
Another opponent of reining in the NSA, Sen. Orin Hatch of Utah, voted yes after saying earlier he'd "like to solve this problem tonight."
Section 215 of the Patriot Act, which authorizes the bulk metadata collection program, is not the only provision of the law set to expire. A key provision allowing counterterror officials to obtain roving wiretaps to listen in on potential terror suspects, even if they change phones, would also lapse.
President Barack Obama has endorsed the USA Freedom Act. In a statement late Sunday night, the White House said the Senate "took an important -- if late -- step forward tonight.
"We call on the Senate to ensure this irresponsible lapse in authorities is as short-lived as possible. On a matter as critical as our national security, individual senators must put aside their partisan motivations and act swiftly. The American people deserve nothing less," it read.
During his weekly radio address Saturday, Obama had urged the Senate to move forward and knocked "a small group of senators" who he said are "standing in the way."
"And unfortunately, some folks are trying to use this debate to score political points. But this shouldn't and can't be about politics. This is a matter of national security," the President said.
While Obama didn't target Paul by name in his radio address, he might as well have.
The Republican presidential hopeful isn't passing up on an ounce of the political benefits his crusade against the NSA's domestic surveillance program is earning him.
He's taken his fight to the campaign trail and on social media, galvanizing his base of support in the lead-up to the crucial Sunday session, all the while his campaign pushed out fundraising appeals.
And a pro-Paul super PAC released an ad Friday that framed Sunday's showdown as a "brawl for liberty," even using the spot as an opportunity to hit not just Obama, but Paul's primary opponent, Texas Sen. Ted Cruz.
Cruz supports the USA Freedom Act, but has argued against allowing the Patriot Act provisions to expire.
And Sen. Ron Wyden, an Oregon Democrat who has largely stood shoulder-to-shoulder with Paul's reform efforts, called for the passage of the USA Freedom Act. Wyden had also voted to move forward on the House bill last week.
But Wyden is also ultimately pinning the blame on the Senate's Republican leadership
"Senate Republican leaders chose to run out the clock until expiration of these provisions was the only likely outcome, and they bear full responsibility for where the Senate stands today," Wyden spokesman Keith Chu said in a statement to CNN.