I'm not a Bernie supporter; that's not hard for me to accept at all. I don't dispute the current polling and support levels. What I question is the notion that public opinion is static and the result of careful, informed analysis of the options by each American. I think most people have no clue whether single payer is better than a public option and are defaulting to the least amount of change because change is scary. When powerful, inspiring voices push M4A, the public gets swept along, and when other powerful voices push back the other way, the public defaults to minimal, incremental disruption.
But the reason that the M4A brand is so strong as to mislead is that Medicare itself is so popular. It is mainly fear of the unknown that makes people reluctant to dive in all the way. That's a real thing that M4A proponents have to deal with, but I don't think the opposition on that basis is written in stone. Leadership means overcoming those fears and getting people to buy into your vision. Which is Warren's challenge right now.