WASHINGTON -- An amendment offered in the Senate Wednesday would block the federal Bureau of Alcohol, Tobacco, Firearms and Explosives from targeting legal gun owners just because they're also medical marijuana patients.

Submitted by Sen. John Walsh (D-Mont.), the amendment to a Senate budgetary bill would ban the ATF from using its funds to criminalize the possession of firearms by medical marijuana patients in the 22 states and the District of Columbia that have legalized marijuana for medical use. The amendment would also protect patients in the states that have legalized CBD oil -- a non-psychoactive ingredient in marijuana that is frequently used to treat epilepsy -- for limited medical use or research.

A 2011 letter issued by the ATF told federal firearm licensees that they are not allowed to sell ammunition or firearms to marijuana users, even if that person is in full compliance with state laws. "[A]ny person who uses or is addicted to marijuana, regardless of whether his or her state has passed legislation authorizing marijuana use for medicinal purposes, is an unlawful user of or addicted to a controlled substance, and is prohibited by federal law from possessing firearms or ammunition," the letter states.

A spokesperson from the Department of Justice said it does not comment on pending legislation.

"Marijuana policy alone already brings together an interesting progressive-libertarian mix of strange political bedfellows," said Marijuana Majority's Tom Angell, commenting on what kind of support the amendment might receive. "When you add in gun rights, we should see a very, very interesting bipartisan result."

Medical marijuana patients found in possession of firearms can be hit with serious federal charges. Take the case of Washington state's Larry Harvey and his family: They face the possibility of decades in prison for growing cannabis, which they use for their own medical needs. When the family home was raided, several guns were found, so the federal government also charged the five defendants with possession of a firearm in furtherance of drug trafficking.

The family insists the guns were used only for hunting in their rural area of the state.

"Larry and I lead a simple life," Rhonda Harvey, his wife, said. "We chop down all of our own firewood so we can lower our electricity bill, and I grow my own fruits and vegetables in the garden that I use for canning and to make pies and jam and stuff. We rely on hunting and fishing to keep ourselves fed throughout the year, which is why we own guns. Now, the federal government is using the guns against us -- all five of us -- and saying that each of us should serve at least five years in prison just for the gun charge alone. It doesn't matter that we used the shotgun to hunt turkeys and the hunting rifle for deer."

Although Washington state legalized medical marijuana more than a decade before the family was indicted in 2013, the federal government still classifies marijuana as a Schedule I substance, a designation the Drug Enforcement Administration reserves for the "most dangerous" drugs with "no currently accepted medical use."

More and more Americans disagree with that assessment. Eight-six percent approve of the legalization of medical marijuana, according to a recent CBS News survey.

"Marijuana is less harmful than alcohol and many prescription drugs, yet the federal government seems to have a persistent case of reefer madness," said Chris Lindsey, legislative analyst for the Marijuana Policy Project, in a statement. "A huge majority of Americans support seriously ill patients’ access to medical marijuana, yet the federal government still treats it like it were radioactive."

A vote on the Senate amendment could come as early as Friday.

Read the text of the amendment: