I don't think that's at all true. Keep in mind regarding Wisconsin that Sanders beat Clinton by 13.5 points in 2016, and won every county except one.
Also keep in mind that the "middle" voter likes the Democrats more on economic issues than on social/cultural issues. Think about how many right-wingers on this forum have said some sort of "I don't agree with Bernie on a lot of things but at least he's not all about 'identity politics,'" or something similar. No, they're not 'concern trolling,' it's that I think that a Democrat that makes economics the core of their message plays better to Democrats' strengths, especially outside of the major urban areas and between the coasts.
Take a look at this video by Clinton's labor secretary. I think it does a good explaining why Progressives have a message that resonates