Handguns could be mailed through the United States Postal Service (USPS) for the first time in nearly 100 years if a proposed Trump administration rule takes effect.

Democratic attorneys general in two dozen states have sent a letter in opposition.

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. The US justice department revisited the 1927 law in January, calling it unconstitutional and arguing that it violated the second amendment to the federal constitution providing Americans the right to bear arms.

The justice department therefore urged the postal service to change its regulations. And it 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”.

In April, the USPS proposed a new rule that would allow anyone to mail concealable firearms like pistols and revolvers. The USPS currently allows some firearms like long-barreled rifles and shotguns to be mailed – but 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 was reviewing public comments – which were due on Monday – before making final changes.

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

“Our state has suffered enough,” Ford said in a statement. “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.”

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 is designed to assist people who are traveling to another state where they might want to use a gun for recreation.

The justice department 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 about two dozen states sent a letter on Monday urging the USPS to withdraw the proposed rule, saying it will make it easier for people who cannot legally possess guns – like people convicted of felonies or domestic violence – to access them. They also said it would make it more difficult to solve gun crimes. They said the executive branch did not have the authority to ignore a law Congress passed, and the rule would 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 were implemented, they argued. There would 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 agencies would 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.

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

The president of Everytown for Gun Safety, John Feinblatt, said the rule change would 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”.