The numbers on single-payer, etc. vary pretty significantly depending on how it's described (seems to be typical with healthcare policy - remember the ACA polling back in ~2010?). Kaiser has an interesting set of graphs from a poll in Jan that try to tease apart some of the distinctions (
https://www.kff.org/slideshow/public...care-coverage/). Their top line, though, is that 56% overall support "Medicare for all", which is how Bernie tries to frame his plan. Depending on how it's described, support can shift around, but it is roughly equally divided. From a quick glance, other polls from the summer/fall show 40-50% support, even when explicitly told it would "replace private insurance" (e.g.
https://fivethirtyeight.com/features...ong-democrats/). Various versions of the "public option" are consistently more popular, though.
I'm not convinced that Bernie's plans are a huge killer to his chances. After all, Trump ran on a number of seemingly-out-there ideas, and there was a lot of talk about taking him seriously but not literally, etc. My amateur electoral college analysis would be that Bernie would be hard-pressed to flip new states to blue (and places like Virginia might get tight), but that he would likely compete well with the Obama-Trump voters in the Rust Belt.
The possible fight with unions mentioned above might complicate that, though.