39 CFR 447.42: Additional prohibited political activities.

(a) In addition to the restrictions on political activities mentioned in § 447.51, an employee may not:

(1) Display a political picture or sticker on property owned or leased by the Postal Service. He is not forbidden by this paragraph, however, from displaying a picture, including a personally autographed picture of a political figure in his office or place of work if it has no language in the nature of political campaigning;

(2) Wear a political badge or button while in uniform or while on duty when that duty requires him to deal with the public or be in the view of the public;

(3) Display a political picture or sticker on his private vehicle while that vehicle is being used for official postal purposes.