That's ridiculous.

Non union
Good luck.

This isn't up to him. It's not his decision to make. You go where the address on the delivery says you go. And how you feel about that matters NOT AT ALL.

He really should quit and go find someplace else to work where he doesn't get to fuck with people's UPS deliveries. Or UPS should fire him.

Question: If somebody's expecting a UPS delivery in his area, and doesn't get it because he decides his opinion is more important, could he be sued? Could UPS be sued?

... that the neighbor is a Democrat. Are they flying big "I'm a Democrat" flags in their yard? Doubtful.

What happens in my neighborhood (we're almost 100% Dems here) if the UPS driver knows I'm not home, the package sits on my porch until I get it and bring it in. If I have to sign for it, they leave a postcard on my door and I go to the UPS office to pick up my package. Nobody needs to know the voting preferences of my neighbors either way.

There is a people search and the first thing that pops up is if you are a democrat or republican and shows the registration of all the neighbors.

candidates of a certain party he apparently doesn't "agree with".

He is refusing to follow a legitimate directive which is insubordination. He gets one verbal warning that it is insubordination and that he can be terminated immediately for it. He does it again, he's gone. Chances are very high that the Teamsters would not waste time defending this as it's in most company policies.