U.S. Senators Bob Menendez and Elizabeth Warren on Wednesday called on federal regulators to issue guidance to speed the adoption of a new merchant category code (MCC) by payment networks to identify firearms sellers.

The letter will add fuel to a debate over the new codes, approved by the Geneva-based International Organization for Standardization (ISO) in September to help detect suspicious firearms and ammunition sales to combat gun violence.

Their implementation was paused by major payment networks including Visa Inc (V.N) and Mastercard Inc (MA.N) last week. The companies cited opposition from Republican politicians working to pass state laws to limit the codes’ use. The Republicans say the codes could be used to improperly track gun purchases.

The MCC would identify the type of store where a consumer shopped, though not the individual items they bought.

In a letter seen by Reuters the two senators, longtime supporters of gun-control efforts, and other Democrats called on U.S. Treasury Secretary Janet Yellen and Attorney General Merrick Garland to “quickly publish any policy advisory, enforcement guidance, and other materials needed by financial institutions, retailers, and law enforcement as the new MCC code is implemented for gun and ammunition stores.”

The letter noted reports that mass shooters had used credit or debit cards to buy the guns and ammunition they used.

In a news release accompanying the letter, the senators noted last week’s implementation pause and wrote that they “believe these credit card companies have a responsibility to push forward, and Treasury and DOJ should provide them, and other financial institutions, with the proper tools for implementation in a timely manner.”

There are more than 40,000 U.S. gun deaths per year, and regulations around guns are again emerging as a major political issue. On Tuesday, U.S. President Joe Biden issued an executive order that reinforced background checks for gun buyers.

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