Or (optimistic version) when consistently voting blue grinds the Republican party out of power so long that Dems split into two viable parties. Can only happen if we do the first part though.

Out of curiosity, do you see Global Warming waiting for the 20 to 30 years that will take in the hypothetical?

Honestly people that want a more left wing party - that's your path. Give Dems the majority in so much that you can have a lefty party run against the Dem without it being a spoiler.