The city relies on a famous-to-Portland protected watershed for its water supply, the Bull Run watershed near Mount Hood. When I lived there in the 1990s, the Forest Service was still trying to log it. Portland voters of all stripes were generally up in arms. People knew that they had great water quality, and the attitude was it wasn't broken, so don't mess with it.
I've looked around the various news sites for exit polls explaining why Portlanders voted down fluoridation by around 60-40. There's plenty of activist reaction that doesn't tell you too much about the typical voter's reasoning, but my best guess is that it's the same reason they opposed logging their water source 20 years ago: it ain't broke.
Maybe a typical Portlander sat down to mark the ballot with limited information beyond knowing that the water system is pretty good as is. With time ranging from five minutes to maybe one hour total over the previous several months, they learn that there are vicious arguments over fluoridation. At the upper range of that spectrum they might learn enough that there's a scientific consensus in favor of fluoridation, with only outlier experts in opposition.
For this amount of information about their water system, and for voters who put in only a few minutes to think about it, the vote against fluoridation isn't irrational. On the other hand, people who spend more than a few minutes on fluoridation should begin to see where the weight of scientific opinion is, and those people are acting irrationally when they overturn an unanimous decision by the city council that they had elected into office, reject what is the clear weight of scientific opinion and then don't put much time into examining the evidence themselves. I'll acknowledge that people who have put in a lot of time examining the evidence could often be anti-fluoride, but I suspect they began as anti-fluoride and then let that interest drive them into examining evidence and being biased in terms of what they accept.
If I'm right about this, the people who put very little time into considering the issue would be anti-fluoride, those who put a moderate amount of time would be somewhat more pro-fluoride, and those who put a lot of time would be all over the map, but quite possibly anti-fluoride and highly motivated.
As to its relevance to climate policy, the one advantage we have is that doing nothing seems like the conservative, do-no-harm option on fluoride, but climate activists have a strong argument against that. Still I think this indicates that we have to have a winning argument for people that spend five minutes thinking about the issue. My best nomination is
Climate change is real. Our modern weather isn't what our grandparents had, what we ourselves experienced in previous decades. You feel it in your bones to be true - that's why the other side is denying it so loudly, trying to overcome what we know is right.Not the most scientific, but not completely unscientific, and maybe it works.