A group of Republican attorneys general are pushing the major payment networks — Visa, Mastercard and American Express — to drop their plans to start tracking sales at gun stores, arguing the plans could infringe on consumer privacy and push legal gun sales out of the mainstream financial network.

The letter comes more than a week after the payment networks said they would adopt the International Organization for Standardization’s new merchant code for sales at gun stores. The merchant code would categorize sales at gun stores not unlike how payment networks categorize sales at airlines, restaurants and department stores.

In their letter, the AGs threaten to use all legal tools at their disposal to stop the payment networks from tracking gun sales. “Categorizing the constitutionally protected right to purchase firearms unfairly singles out law-abiding merchants and consumers alike,” the letter said.

The payment networks said when they adopted the policy that they are just following the guidance from ISO. It will be largely up to the banks who issue the credit and debit cards to decide whether they want to stop sales under certain merchant codes.

The CEOs of the major banks will appear in front of Congress today and Thursday, and they are likely to be asked questions on the gun store sales tracking controversy.