Almost all Democrats since Al Gore's Inconvenient Truth (and many before) have supported various levels of action on climate change. "Mainstream" Democrats and "socialist" Democrats agree like 99% on policy. The differences are primarily cultural and trumped up by Republicans and the media.

I don't know of a mainstream Democrat who doesn't support mainstream policies in conservation, the Paris Climate Accord, fuel efficiency standards, environmental research and protection, climate science, etc. These are all things the current GOP is taking real steps backwards on, so mainstream D vs the majority of Rs doesn't even come close. And in general or national elections you don't have much of a choice but to support the mainstream candidate who comes closest to a sane environmental choice.