Three of the country’s largest credit card processors have quietly resumed work on implementing a special merchant code for gun stores.

Credit card networks Visa, Mastercard, and American Express are currently working to comply with a new California law requiring payment processors to implement a specialized Merchant Category Code (MCC) for transactions at gun and ammunition retailers, CBS News reported Monday. Golden State lawmakers passed the requirement, which takes effect in May of 2025, with the hopes of prompting financial companies to track and ultimately flag “suspicious” purchases for law enforcement.

The news marks a significant policy reversal by the leading credit card companies. All three announced that they were suspending any work related to creating and implementing an MCC for gun businesses last March after the project drew public backlash from Republican lawmakers and inspired new state laws banning gun store MCCs. The decision by the companies to resume work on gun store MCCs for California retailers will almost certainly breathe new life into the political battle over the codes and domestic gun policy.

The three companies signaled their intentions to comply with California’s MCC requirement in an unpublicized letter to a group of congressional Democrats, according to CBS News. Executives from each company authored the letter in response to an inquiry launched by Senator Elizabeth Warren (D.) last December probing the companies’ rationale for pausing work on gun store MCCs.

While the letter noted their willingness to comply with California law, it also featured comments from the executives expressing their continued discomfort with the political climate surrounding the code.

“With respect to the [firearm merchant code], there continues to be a tremendous amount of regulatory and legislative uncertainty,” Visa Senior Vice President Robert B. Thomson III wrote.

At least seven Republican-controlled states, including sizeable states like Texas and Florida, have passed legislation outlawing the use of firearms retailer tracking codes. Around a dozen additional states are currently considering similar legislation. Meanwhile, Colorado lawmakers are currently advancing a bill that is nearly identical to California’s MCC mandate.

News of the three companies’ work on implementing the California gun store MCC was met with strong reactions from advocates on both sides of the gun debate. Kris Brown, President of Brady and a leading proponent of gun store MCCs, responded positively to the news and urged more states to follow suit.

“Our current patchwork of laws across the U.S. is making it difficult for banks and credit card companies to navigate,” she said. “We need to make sure these codes are being used in every state that they can be, and pass federal legislation to put public safety over gun industry profits.”

Meanwhile, gun-rights advocates criticized the implementation of the code in California and called on federal lawmakers to put a stop to them once and for all.

“Congress must bar the use of this gun owner privacy-invading Code,” Larry Keane, Senior Vice President of the National Shooting Sports Foundation, said. “It will do nothing to stop crime.”

The emergence of MCCs as a significant political issue stems from a 2018 New York Times article that found the majority of mass shooters between 2007 and 2018 used credit or debit cards to purchase the guns and ammunition used in their attacks. Gun-control advocates have since argued that attaching MCCs to gun stores could allow payment processors to identify “suspicious” gun purchases for law enforcement—though they have thus far offered little detail on what the threshold should be for suspicion.

Gun-rights advocates have countered that the code could be used to chill legal firearms commerce by making payment processors less willing to facilitate gun sales. They also argue that the code would amount to a de facto private gun registry and violate gun owners’ privacy rights.

The argument was largely theoretical until the International Organization for Standardization (ISO), a voluntary membership organization, announced it had created an MCC for gun retailers in September of 2022 after facing years of pressure from gun-control advocates and activist financial institutions, such as Amalgamated Bank. Each of the country’s largest credit card companies originally agreed to adopt the gun store MCC across the country before Republican-led backlash prompted them to take a step back.

Visa, Mastercard, and American Express did not respond to a request for comment.