@ehBeth,
Of course he has a right to refuse this kind of business but rest assured that if a gun store owner who openly supported Obama had a similar policy against Romney voters, I guarantee this gun shop owner and a few million gun nuts would be screaming "Obama is trying to take your guns away conspiracy. This refusal to sell guns to Romney supporters is proof! (Etc...)"
That sentiment is basically the raison d'etre of this thread. Militant gun enthusiasts tend to fail to notice their own hypocrisy. Where any curbing of anyone's gun rights is a grand cause for protest, then why isn't this curbing of a small group of gun enthusiasts a similar reason for protest?