In 1927, Congress passed a law barring the USPS from mailing concealable firearms unless they were from licensed dealers in an effort to curb crime. In January, the Department of Justice revisited the 1927 law, calling it unconstitutional and arguing that it violated the Second Amendment, and urged the postal service to change its regulations.

The Department of Justice said that as long as Congress chooses to run a parcel service, “the Second Amendment precludes it from refusing to ship constitutionally protected firearms to and from law-abiding citizens, even if they are not licensed manufacturers or dealers.”

Last month USPS proposed a new rule that would allow anyone to mail concealable firearms like pistols and revolvers. USPS currently allows some firearms like long-barreled rifles and shotguns to be mailed, however they must be unloaded and securely packaged. Similar protections would be in place for handguns, which have evolved since 1927. The USPS said in a statement that it is reviewing public comments — which were due Monday — before making final changes.

Nevada Attorney General Aaron Ford, a Democrat who is running for governor, said the rule change would undo the work states like Nevada have done to curb gun violence. Nevada experienced the deadliest mass shooting in modern U.S. history, when a gunman on Oct. 1, 2017 opened fire from the Mandalay Bay casino hotel in Las Vegas, killing 60 people. Following the shooting, Nevada passed a law requiring state-administered background checks on most private gun sales or transfers.

“Our state has suffered enough, and to suggest we make it easier for criminals and abusers to access firearms is a slap in the face to gun violence survivors and law enforcement,” Ford said in a statement.

Under the proposed rules, someone could sell and ship a gun to a person within state lines. The rules are tighter for mailing guns across state lines — people could only mail it to themselves in the care of another person and would be required to open it themselves. That’s designed to assist people who are traveling to another state where they might want to use a gun for recreation.

The Department of Justice argues the patchwork of state laws around guns makes it difficult to take them across state lines for lawful purposes like target shooting, hunting and self-defense. It said that in many cases, people have no ability to travel with a firearm, making mail the “only viable method of transportation.”

Ford and other attorneys general in around two dozen states sent a letter Monday urging the USPS to withdraw the proposed rule, saying it will make it easier for people who can’t legally possess guns, like people convicted of felonies or domestic violence, to access them. They also said it’ll make it more difficult to solve gun crimes. They said the executive branch does not have the authority to ignore a law Congress passed and the rule will override state gun laws.

State laws include requirements like firearms safety courses, background searches and mental health history checks, according to the attorneys general. Those requirements are regulated through state entities, which would be bypassed if the rule change would be implemented, they argued. There will be no way to guarantee that someone is following the rules and not shipping a handgun across state lines to another person, they argued.

Law enforcement will have to create a new tracking structure to account for firearms mailed through the postal service, which would place added burdens on state budgets, the attorneys general said.

Private companies like UPS and FedEx also restrict gun shipments to customers with federal firearms licenses, such as importers, manufacturers, dealers and collectors. FedEx requires shippers with a federal firearms license to work with a FedEx account executive to obtain approval, according to the company’s website.

Firearm advocacy groups applauded the proposed change, while gun safety organizations expressed their concern.

John Commerford, executive director of the lobbying arm of the National Rifle Association of America, called it a key victory for law-abiding gun owners.

“Thanks to President Trump and his administration, USPS will finally allow these firearms to be shipped under the same commonsense safety conditions as rifles and shotguns,” Commerford said in a Wednesday statement.

John Feinblatt, president of Everytown for Gun Safety, said the rule change will turn USPS into a “gun trafficking pipeline” for illegal weapons “while stripping law enforcement of the tools they need to prevent and investigate gun crime.”