I don't think Sanders will get anywhere. I think he got his boost from his die hard followers when he announced but I really cant see him doing well. I dont know anyone who voted for him last time who's planning on voting for him again. There's other better progressive options.Beto O’Rourke looks an interesting proposition, he could be a real dark horse in the run in.
I still think Sanders would be the most credible threat to Trump though
Don't get me wrong, we should be grateful that he put so many policies in the spotlight but by the time the primaries roll round he'll be poling fourth or fifth. Independents don't want to replace one extremest with what they perceive as another.
By the time the primaries roll round he'll be polling behind Biden, O'Rourke, Harris, Buttigieg and possibly Warren.