The United States Postal Service isn't backing down from its ban on marijuana advertising in the mail. Instead, it's expanding the embargo.

Last month, the Portland District of the U.S. Postal Service sent a letter to the Chinook Observer newspaper in Long Beach, Wash., saying it's illegal to mail a newspaper with medical or recreational weed ads.

The Oregon congressional delegation, led by Sen. Ron Wyden, peppered the Post Office with questions, including where it found the authority to ban the mailing of marijuana ads, and whether that policy was in place in all 50 states.

The Postal Service's attorney wrote back to Wyden on Dec. 15 with an answer: The ban hadn't been formalized nationwide. But it is now.

"To insure uniform understanding of this issue," general counsel Thomas J. Marshall wrote, "we are issuing the following national policy that will apply to all districts."

That policy says that if postmasters receive an item to mail that contains weed ads, they should send a warning to the person or company mailing it. The Post Office can't keep the item out of the mail, the policy says, but it can report the violation "to the responsible law enforcement agencies."

Marshall also explained where the Postal Service got its power: from Congress, which continues to treat marijuana as a Schedule 1 drug.

The ban could have a chilling effect on newspapers that deliver by mail. The Chinook Observer has already announced it's removing pot ads from the issues it mails. (WW delivers a small amount of papers by mail.)

Wyden's office issued the following statement this morning (in league with Sen. Jeff Merkley and Reps. Earl Blumenauer and Suzanne Bonamici):

“We are working as a delegation to quickly find the best option to address this agency’s intransigence. We want federal authorities to respect decisions made by law-abiding Oregonians and small business owners in the state.

Unfortunately, the outdated federal approach to marijuana as described in the response from the Postal Service undermines and threatens news publications that choose to accept advertising from legal marijuana businesses in Oregon and other states where voters also have freely decided to legalize marijuana.”