First things first: everyone in a town is negatively affected by a post office closure, therefore everyone in the town is a potential ally. Individual political concerns (conservative, liberal, progressive, pro-gun/anti-gun, &c.) have nothing to do with fighting to keep a post office open.
Some people who would otherwise be intractable political rivals can be invaluable allies in a post office fight: use every last one of them. Our most important ally in our fight was the gun shop owner here in town.
On saving a Post Office: organisation is key. You must organise and prepare to rebut every document issued by the Postal Service. (If you are not good at accounting then finding an accountant who will do the work of forensic accounting on the Post Office’s paperwork is vital.)
We first obtained a copy of the first closure study. (Your postmaster has it, but is not permitted to show it to you unless you ask.)
Once you get that closure study, make a copy of it to take home. (The Post Office mayn’t let it out of the office but can copy it for you if you ask, though there might be a charge for the service.)
Then go over every single issue in the study with a fine-toothed comb. We also obtained the assistance of the three postal unions and the Retired Postmasters Association. We made sure they got copies of the closure study, sent by Certified Mail with Return Receipt (to ensure they were not lost in the mail).
Read more: How We Saved Our Rural Post Office.