There could be big implications if Congress lets a hotly contested post-9/11 law die

The Patriot Act as we know it is in trouble.

Passed in the wake of the September 11 terrorist attacks, the act has been used to justify controversial national security programs - including the National Security Agency's bulk collection and storage of Americans' call records.

Before the act's expiration on June 1, a coalition of Democrats and a few libertarian-minded Republicans are threatening to block renewal efforts unless the NSA's dragnet spying program is significantly curtailed.

If Congress fails to reauthorize the act, the NSA would be forced to shutter the controversial program.

The NSA has already begun winding down the program. On Wednesday, the Justice Department circulated a memo warning lawmakers that the NSA had begun shutting down the program in preparation for the law's expiration on June 1.

But the expiration would also have wider implications.

The FBI would be hit hard by a failure to reauthorize. The agency would no longer be able to operate "roving wiretaps," which allow the agency to quickly change wiretaps when a suspect switches phones, alerting courts after the agency has tapped a new phone, as The New York Times has reported.

As Politico notes, the agency would also lose access to certain private records, which include suspected terrorists' library reading lists and gun ownership data.

Property searches would be much harder. The agency can currently obtain a search warrant for a suspected "lone wolf" terrorist even if that individual has no proven links to an official terrorist organization. Agents would also be barred from searching private properties without notifying owners until long after agents have left, according to the ACLU.

Reauthorization remains tied up in the Senate, where a coalition of Senators want to revise Section 215 - referred to as the "business records provision" - which the Obama administration has used to justify the NSA's bulk collection of telephone metadata.

The likelihood that Congress won't reauthorize any part of the Patriot Act is unlikely. The House has already overwhelmingly passed the USA Freedom Act, which keeps most of the Patriot Act in place but puts some restrictions on the NSA's telephone collection operations.

Already several compromises are also in the works to save the Patriot Act, including a short-term reauthorization pushed by Sen. Majority Leader Mitch McConnell (R-Kentucky) that would give Congress more time to debate NSA spying after a brief recess.

But even if Congress does reauthorize the Patriot Act, there's no guarantee that the most controversial parts will survive. Earlier this month, a US appeals court ruled that bulk collection of data was not covered under Section 215.

"The government can point to no grand jury subpoena that is remotely comparable to the real-time data collection undertaken under this program," the opinion stated.

That could set up a showdown at the Supreme Court.

"This was a surprising decision given the reluctance of courts to second-guess intelligence-gathering in the war on terror," UCLA law professor Adam Winkler told Business Insider. "Given the importance of this ruling for national security, the Supreme Court is likely to step in."