It's all about turnout like usual for Dems as far as down ballot races go. Can his popularity with young people and Independents who usually don't vote where policies like marijuana legalization, medicare for all, and college debt relief are popular turn out more people at a higher rate of more moderate democratic voters he may lose? It remains to be seen if he is the nominee. Nobody really knows until election day unfortunately.
You really think Democrats in Congress would allow that? At best he somehow gets a public option through.