I think you're missing the point.
If someone comes at me, attempting to persuade me that their side is the right one, but their persuasion style is to be adversarial and say I'm a shithead if I don't pick their side, I am more likely to reject their side.
This has little to do with the "facts are stronger" argument. I agree that many people do not pay attention to objective facts, and instead make decisions based on emotional factors. And that's exactly what would be happening here: this type of adversarial argument would give me a bad feeling, and I would reject it.
Beyond that, you can still make emotional arguments that use facts as their foundation. I would respond much better to something like "75% of your co-workers have shit wages, and a union can help them get fair pay; don't you want the best for your colleagues?" That sort of argument isn't all positive, as it tries to make the other person feel guilty, but it's a much better argument than "if you don't want to join the union you're an asshole".
> I think it's fairly naive to think that "we developed [our opinions regarding unions] over time by deciding on the arguments advanced," as though it were always a conscious process or that a broadly fair view of unions was widely disseminated with which to make an informed decision.
I don't think anyone's claiming it's a rational, conscious process. Very much of it can be emotional and subconscious. But the result is still the same: if you tell me that I have to "support the union or else", I will -- consciously and rationally, or subconsciously and emotionally: it doesn't matter -- gradually develop a negative opinion about unions.
I think this is the trap a lot of people (myself included, sometimes) fall into: they want people to evaluate their arguments on their terms, how they themselves would evaluate them if they were the recipient of them. But that's not how it works. People evaluate arguments in their own way (regardless of whether the argument is intended to evoke a rational or emotional response). If you want to make an argument, you need to focus on outcomes, not on your own personal preferences as to how things should turn out. Tailor your argument to the recipient. Sure, for some people the "you're either for us or against us" type argument might be the most effective. But it's not for everyone, and it can alienate people as well.