"If you vote for _____ we can't be friends" for me means, "we can't be friends" regardless of who's in the blank.
Political beliefs are such a divisive area. On the one hand it would be nice if everyone could have their own political beliefs and still get along. On the other hand those political beliefs are, more often than not, an indirect way of telling other people how to live their life without actually going up to that person and saying "I disagree with what you're doing, and I'm not going to let you do it"
My personal belief is live and let live, and be honest and direct about whatever it is you want or do. As long as no one is being forced to do something then they can do whatever they want. If we could just get that right then I think so many other issues would resolve themselves. Unfortunately, that's not how a majority of other people believe, and often their beliefs limit my beliefs.
While I agree with the softer side of the sentiment of the original statement, I also disagree with how final the statement is. But then I guess that depends on what you call a friend, and each person has a right to define that how they want to. We may not all agree on what exactly a friend is, but that's part of what makes, or doesn't, someone a friend.
I consider most people acquaintances, for various reasons. We don't have much in common or to relate on, they have political beliefs that limit my choices, we don't actually hang out or do much together (if any), we don't talk much, I don't think they'd be there for me if something went horribly wrong, they don't accept me for who I am, they're always busy with their own circles that are mutually exclusive, etc. Not all of these are required, as I still consider some people friends who do go against some of them, but they make up for it in other ways.