Here's one thing people need to understand about polls, regardless of their methodology--whether they're phone interviews, written surveys, online surveys, in-person canvassing, etc. The people who participate in polls are self selecting. Even if the population samples are well chosen (e.g., zip code, gender, ethnicity, income, etc.) and statistically valid, the respondents are always people who are willing and interested to participate. Right now, Republicans and others who support Trump have zero motivation to participate in polls, which are primarily asking people which Democratic candidate they prefer. Trump supporters have always been underrepresented in polls. Even polls taken within 36 hours of the last election and exit polls taken on election day were predicting Hillary Clinton would win the last election handily.
Current polls that project candidates like Sanders, Harris or Buttigieg beating Trump in a general election are clearly not including Trump supporters in their samples. They're primarily polls that target Democrats for the purpose of stack-ranking the Democrat candidates.
Dems are going to need a strong candidate to beat Trump in Fla, NC, Ohio and Penn. I don't think they have one who can do it, at least not yet. I don't think either Biden or Sanders can carry a national election, one because he's too uninspiring, and the other because he's too radical. I have doubts about Warren as well.
