climate change (a tail end existential risk

What kind of scenario are you thinking of when you argue that climate change is an existential risk? How do you think it might kill all or even 90% of the population?

Democrats (and parties of the left) are since they're more likely to favor policies which reduce the threat of climate change

While the Obama administration did a few symbolic actions for climate change it didn't move significantly on the issue. I don't think there good reason to assume that things would be different under another Clinton.

Nixon went to China and the Obama administration waged it's war against whistleblowers. There might be more political room for a Republican government to make substantial action on climate change than for a Democrat government.