Federal law prohibits contributions by corporations (includes banks). The OCC explicitly addresses the contribution issue for their supervised institutions in
Bulletin 2000-8. I've never heard of anything that attempts to address the posting of signs or communication per se. For state campaigns there may be state rules, but I doubt that an enforceable prohibition could be fashioned, so I would guess the matter has been left to good taste and good judgment. You might test this theory by calling your state's department of banking and election commission.
The bank has three "publics" to consider in the public relations impact of advocating a candidate: its customers, its employees, and the other candidates. Offending one or both of the first two would be my overriding concern...