So, in your mind, if someone did this favorability poll you want, and it showed, say, 60% favorability for the Green Party, you would vote for them, and you imagine that the majority of Democratic voters would all spontaneous switch their votes over together?
Not Democratic voters (assuming you mean the party). Just voters.
If you're a Democrat and you feel like the Green Party has a candidate polling at a majority that represents your interests more than the Democratic candidate, why would you vote for the Democratic candidate instead? It goes against your interests. I know some Democrats are brain damaged, but I think that is only a small percentage (1 - 3 %).
This is like saying the majority of the population is leftist and has a chance at a bloodless revolution, but they decide to not take it because of shits and giggles.