New York CNN  — 

Two dozen Republican attorneys general warned Visa, American Express and MasterCard on Tuesday not to go ahead with their plans to add a new merchant category code for gun retailers.

In a letter to the CEOs of the card giants, the officials expressed serious concern that adopting a new sales code for gun stores would harm the constitutional rights of gunowners and potentially violate consumer protection and antitrust laws.

The Republican attorneys general are threatening to take legal action against the card companies if they follow through on their recently-announced plans to adopt the new code, which was approved by an international group that sets payments standards.

“Be advised that we will marshal the full scope of our lawful authority to protect our citizens and consumers from unlawful attempts to undermine their constitutional rights,” the letter reads. “Please keep that in mind as you consider whether to proceed with adopting and implementing this Merchant Category Code.”

Advocates for the new code argue it could potentially flag mass shooters and gun traffickers.

Previously, gun store sales were classified under a general merchandise or sporting goods category. These codes show where people make purchases but don’t detail what they specifically bought.

The GOP letter, first reported by The Wall Street Journal, argues the new code for gun stores is the result of “transnational collusion between large corporations leveraging their market power to further their owners’ desired social outcomes.”

The Republican officials, led by Tennessee Attorney General Jonathan Skrmetti and Montana Attorney General Austin Knudsen, argue the effort will result in “vague and misleading” information.

“This categorization would not recognize the difference, for example, between the purchase of a gun safe and a fire arm,” the officials wrote, adding that gun purchases at department stores would not be captured.

However, the credit card companies have said they will continue to support lawful purchases and that won’t change by adopting the new standards. In other words, legal gun purchases won’t be blocked by this new merchant category code, or MCC.

“We do not believe private companies should serve as moral arbiters,” Visa said in a blog post last week. “Asking private companies to decide what legal products or services can or cannot be bought and from what store sets a dangerous precedent.”

Visa added that its network doesn’t allow financial institution members to deny transactions for the purchase of legal goods or services based on what merchant code they fall under.