When is a ban not a ban? When it might unintentionally inspire a practice it attempts to outlaw.

Last week, Democratic Sens. Cory Booker and Jeff Merkley introduced the Ethical Use of Facial Recognition Act, which seems to suggest at least a temporary "moratorium" on the tech.

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"I'm basically trying to take on the federal government from being a Big Brother government that tracks Americans everywhere you go," Merkley said in a Wednesday interview with Recode. "It's a technology that's spreading very, very quickly, and it has huge implications for privacy and for the power of government. And I think we should hit the pause button."

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To be sure, the bill does block federal funding for facial recognition systems until a bipartisan commission recommends, and Congress acts on, rules that consider issues such as protecting civil liberties by reducing racially biased error rates — a known problem with the technology.

Andrew Guthrie Ferguson, a law professor at University of the District of Columbia who testified before Congress about facial recognition tech last year, called the bill a "great first step."

"The idea of a formal pause and an official commission makes a great deal of sense in the current informal and ad hoc environment," he said.

But the bill only applies to federal use of the technology. And even then, it also includes a huge carve out: Federal officials could still use facial recognition technology if they get a warrant. And some experts warn it could end up actually expanding the use of the technology by the federal law enforcement.

"While the warrant requirement appears to be a good baseline in theory, in practice its implementation will not just allow but, more importantly, incentivize the continued expansion of facial surveillance infrastructure," said Evan Selinger, a philosophy professor at the Rochester Institute of Technology. "The expansion will drive mission creep and, eventually, leave society in the regretful position of the metaphorical slowly boiling frog."

Farhang Heydari, executive director of the Policing Project at NYU's School of Law, also has questions about how such warrants would work practically, which the bill in its current form does not fully describe. "What is it that police would need to show? That they have probable cause to believe the unidentified individual committed a crime?" he asked.

Even Ferguson, who advocates for a warrant requirement in some uses, is worried about the carve out in this bill.

"I think it would allow face surveillance and face tracking with a warrant, which might be more permissive than they would want to grant," he told Protocol in an email. "It is one thing to require a warrant for face identification, but to allow generalized face surveillance (even with a warrant) would be quite an expansion of existing use."

Facial recognition technology has multiple use cases. Identification, like when you run an image of a suspect against a database to for a match, is generally considered less invasive than real-time surveillance or tracking where everyone in a place might be identified and logged.

Neither Booker nor Merkley responded to a request for comment at the time of publishing.

Pushback against facial recognition has grown more acute than ever in recent weeks. One facial recognition provider, Clearview AI, offers an app built on billions of images scraped from the internet and has been used by hundreds of police departments around the country, The New York Times reported in January. The backlash to that revelation was swift: New Jersey's attorney general announced he was barring state prosecutors from using the app, and a coalition of 40 organizations petitioned the Privacy and Civil Liberties Oversight Board to rein in government use of facial recognition technology.

Meanwhile, UCLA on Wednesday canceled a plan to use a facial-recognition security system on its campus, after students and privacy advocates spoke out against it. And all this pressure follows a groundswell in local stands being taken against the technology. In some cases, cities, including in San Francisco and Cambridge, have enacted local bans on facial recognition being used by the police and other city agencies.

Even with the carve out concerns, the legislation proposed by Booker and Merkley is still one of the more aggressive legislative attempts to take on facial recognition. It goes further, for example, than a bill introduced in November by Sens. Christopher Coons and Mike Lee, which proposed a structure requiring court orders for public surveillance, but with its own carve out for "exigent circumstances."

Perhaps a bigger problem for any of these calls for making federal use of facial recognition technology more difficult is that Congress passing, and President Trump signing, the bill seems unlikely.