Here's the thing: no one gets to choose who is the candidate of the poor and disenfranchised except the poor and disenfranchised. If they don't think he speaks for them, then he doesn't. He may think he does. We may think he does. But if they don't then he doesn't. And if he doesn't see that as a problem he needs to address, then it becomes a big problem.
By the way, Sanders, Clinton, O'Malley, Bush, and Carson spoke at the National Urban League Convention about a week ago. Clinton was the only candidate that drew more than a polite response from the audience. It was Sanders' first appearance at the convention (I think).
"It does me no injury for my neighbour to say there are 20 gods or no god. It neither picks my pocket, nor breaks my leg."
-- Thomas Jefferson