While you are on liberty, you can wear a Ron Paul shirt. While you are out in town, you can campaign for Dr. Paul. You CANNOT endorse Dr. Paul while IN uniform, you CANNOT send campaign e-mails from your .mil address, and you CANNOT say (if active duty/reserve/IRR) that you are such and such rank of X branch of military, and that you support Dr. Paul.
Different bases have their own regulations as well. I know that on some USMC bases, people are not authorized to put signs in their yard for any candidate, while on others they are. I think a fact-finding and regulation hunt is in order. We need to find out what regulations each service has, and then what regulations each individual post has. Most bases have a website nowadays, and oftentimes have their rules and regulations online in PDF form (it is this way for most USMC bases).
I'm a Marine veteran, and I support Dr. Paul.