the two major political parties in the US (...) people think is most likely to reduce existential risk

No comment on the main question, but if you really care about an issue you should try like hell to prevent it from becoming a wedge issue. There's no longer any meaningful discussion of AGW in the US, because it's now a wedge issue. Even if you observe a huge correlation between political tribal affiliation and getting the "right answer", you should never point this out. Once people start to absorb their position on a topic into their self-image, they will never change their minds about it.