1. He leads in a lot of polls in 47 states. The moderate dem diehards will vote for whoever has (D) next to their name on the ballot, there's no need to worry about their support.
2. Bernie isn't going to lose on the basis of Brexit and an anti-semitism media scandal. It's a false equivalency, the outcome isn't determined just because they both happen to be on the left in countries with different politics. As much as I am a supporter of Corbyn ideologically, it is clear that Bernie is the better communicator on issues of solidarity and can relate to the problems of the working class, regardless of their identity (this means immigrants and non-whites, who are large parts of the dem voting bloc).
https://www.youtube.com/watch?v=6fXPvA2MKi0
I've heard him say this a few times but it still really strikes me when I hear it. He totally gets people's frustrations in a way none of the others do. He understands why people bought into Trump so much, but he can link it to social issues rather than lies, bigotry and scapegoating. I'm not saying he'd be the best candidate in every election but this one? He is the only one who makes sense. Like please make a convincing argument for any of the other candidates on how they could defeat Trump electorally but also culturally, I'm all ears there because I haven't heard any yet.