In stead of posting a sign that says "no trump supporters", we can make them identify themselves. Sounds like an idea for a new web service.

I would put no such sign on my door. It could make you a target for lunatics.
Putting yourself at risk is not going to educate or change anyone's mind. It's football team mentality with them.

Totally get it, but food for thought: he's not as set in his ways as his parents. He probably doesnt agree with them as much as he feels like he has to show. Pick his brain, ask him to explain WHY he himself feels that way. If he doesnt think he has a good answer, maybe he'll start to shift a bit.