When the clock struck midnight on June 1, the National Security Agency’s sweeping call records surveillance program ended.

The expiration was a major loss for Senate Majority Leader Mitch McConnell of Kentucky. He wanted the Patriot Act's Section 215 that authorized the spying extended without any change. But it was a coup for Senator McConnell's fellow Republican senator from Kentucky. Sen. Rand Paul successfully stood in the way of renewing the bulk collection that vacuumed up Americans’ call data, clearing the way for Congress to pass a landmark surveillance reform bill instead.

While the fight over Section 215 aired deep divisions in Congress over surveillance practices, behind the scenes, an unusual alliance that brought together far-left Democrats and conservative Republicans had been fighting hard against a clean reauthorization of the Patriot Act and in favor of surveillance reforms. Now, that coalition is celebrating the Patriot Act's brief expiration as its first major victory – in what the group pledges will be a much longer fight to protect civil liberties in the face of what it considers to be excessive government surveillance.

“It’s a real game changer," says Becky Bond, political director of progressive activist organization Credo Action. “We found ourselves aligned fighting for the full repeal of the Patriot Act. Since we don’t align on many other issues, the mission was very focused.”

The loose-knit network, dubbed the Civil Liberties Coalition, spanned all corners of the political spectrum and united groups such as Credo and conservative advocacy group FreedomWorks that are usually at odds on other hot political issues. Its diversity is a rarity in Washington as it was focused on a single issue – ending mass surveillance – while at the same time not seeking the limelight media attention. Some members are even reluctant to be publicly affiliated with each other because they have such polar opposite views on issue such as health care and economic policy.

"Alone, libertarians nor progressives have not been able to make much headway," says Ms. Bond. Now, she adds, "there’s a recognition that progressive and libertarians coming together can form a very powerful counterweight to the White House, security agencies, and even leadership in Congress when we line up. We have some power, where people felt really powerless before.”

Government surveillance proved to be a common enemy that transcends traditional political ideologies, adds Josh Withrow, legislative affairs manager at FreedomWorks.

“Regardless of what you think about social welfare, spending policy, and the national debt, everybody is affected pretty much the same way when the government starts collecting records and surveilling you and violating that essential right to privacy that I think most Americans agree is a natural and constitutional right,” Mr. Withrow says.

The network began to jell in the months leading up to the June 1 expiration of Section 215 and as surveillance reform bills began to gain traction in Congress. It became clear to many activists and privacy hawks in Washington that they'd need to form a broader coalition to mount an effective battle to reform government surveillance practices, the scope of which ex-NSA contractor Edward Snowden revealed in leaks that began two years ago, to defeat powerful national security hawks such as McConnell.

"This coalition came together and thought, ‘Let’s flex our muscle and see what happens,' " says Sascha Meinrath, founder of tech policy think tank X-Lab who also serves as a coordinator of the Civil Liberties Coalition. “It turned out we were able to do a lot more than we thought.”

For weeks leading up to the Patriot Act vote, the Civil Liberties Coalition lobbied lawmakers directly, pledged millions of dollars in financial support to candidates from both parties to raise the profile of surveillance as a political issue, and urged their combined membership bases of tens of millions of people to write or call congressional offices to support reforming surveillance laws.

Most of the privacy advocates agreed the USA Freedom Act – the bill President Obama eventually signed to end bulk collection and force the agency to collect the data from the private sector – would be better than the straight extension of the spying authorities McConnell was proposing, but many thought it did not go far enough.

So a critical mass within the coalition formed what David Segal, executive director of progressive political activist group Demand Progress, called a "vanguard, grassroots flank" pushing for a total sunset of the Patriot Act provisions. "We think USA Freedom is woefully insufficient at best," Mr. Segal said.

Yet Segal and other advocates acknowledged that, if the coalition could convince a swath of members in both parties to hold out for sunset, lawmakers against surveillance reform would be forced take it seriously. After all, if no compromise was reached and the authorities were about to expire, the coalition believed – correctly – that Congress would be more likely to have USA Freedom form the compromise than a weaker surveillance reform bill.

The group generated at least 600,000 e-mails to lawmakers’ offices and tens of thousands of phone calls in the weeks leading up to the vote.

What's more, during the intense days of the Patriot Act fight, the various segments of the groups targeted the members of Congress naturally more receptive to their core issues. “We could lead with the Quakers, or [progressive digital rights advocacy group] Fight for the Future, or FreedomWorks – in a way that traditional coalitions of this sort just don’t do, because they are coalitions of either the left or right,” Meinrath says. Each member of the coalition found allies on Capitol Hill who advocated for reforming Section 215, making for strange bedfellows in Congress. Republican libertarians such as Senator Paul or Rep. Justin Amash found themselves in the same camp as liberal Democrats such as Sen. Ron Wyden of Oregon and Rep. Mark Pocan.

With the passage of the USA Freedom Act behind them, the alliance is already working on a multiyear, coordinated strategy to push Congress to continue surveillance reforms.

Just last week, the House of Representatives approved several antispying measures within a major appropriations bill – including banning the government from forcing tech companies to place “back doors” into encrypted devices, and prohibiting the Drug Enforcement Agency from revamping a once-secret bulk call records collection program.

The coalition supported and consulted with more than 50 House lawmakers on these efforts, and is looking ahead to the Defense Appropriations bill expected later this month as another vehicle for anti-spying amendments. “You’ll see an immediate push for amendments to appropriations bills to prevent the NSA from making use of what’s called the ‘back door loophole’ for warrantless wiretapping of Americans,” Segal said. The coalition is eyeing several amendments, including one that would prevent the NSA from purposefully degrading the security of devices to get access to the data.

The coalition is also looking ahead to 2017. That's the expiration data the Patriot Act's Section 702, a provision that allows the collection of foreign data but Mr. Snowden's revelations showed also sucked up Americans' e-mails and chats without a warrant. The coalition is working with lawmakers on amendments to effectively defund the spying practices by stipulating no money be allocated toward them. "We don’t have to wait until 2017 to wait for Section 702 to expire to demand attention for greater reforms,” Withrow said. “All we have to do is keep on encouraging them, and letting them know there’s a grassroots constituency outside the beltway that has their back.”

Some in the coalition are working with lawmakers on various strategies to create an oversight and investigation panel, analogous to the Church and Pike Committees of the 1970s, to vet the totality of surveillance activities in the post-9/11 era. Others want to see Congress debate whether Director of National Intelligence James Clapper should keep his job – who, depending on who you ask, either lied or misspoke when he testified the NSA does not wittingly collect data on millions of Americans.

All this, too, could have an impact in election season. FreedomWorks, for instance, generated 10,000 phone calls from conservatives in the weeks leading up to the Patriot Act vote to ensure McConnell heard from the right wing of his party that a clean reauthorization was “absolutely unacceptable” and “toxic to him, his party, and allies,” says Withrow.

Now that the group knows its base is energized on the issue of privacy, it won’t stop there. FreedomWorks also has a political action committee that raises money for and endorses Republican candidates – and plans to call out conservatives who are, in its view, failing on surveillance and civil liberties issues. “We need to make sure conservatives are aware of which Republicans are good on this issue," Withrow says, "and which are bad."