Bernie got booed.
He dropped a few names and got booed. Take a look. It's not long.
https://www.youtube.com/watch?v=xAXzS9I0zHc
I was surprised. After watching it, I'm not sure why they booed.
I believe the audience wanted him to talk about how to fight
against white nationalists. Bernie's platform focuses more on how prevent people from
becoming white nationalists so he didn't have the answer the audience wanted, and so he got booed. In other words, he got booed for what he didn't say, not for what he did.
You can see the Andrew Yang struggle a bit with very much the same question in his town hall here:
https://youtu.be/5Q6sDKvwdO0?t=180 Essentially both Yang and Sanders's platforms are based on the assertion that the actual problem we face is rising tribalism in the US. If that is indeed the problem, the solution is to identify and fight the roots of that change in our society. Roots like growing economic insecurity, which we do know from countless studies makes people act more racist and hateful. If we identify and confront those roots, fewer people will become radicalized supporters of hateful ideologies. So Sanders focuses on questions of inequality rather than race, and Yang focuses on economic fixes to social instability (like the freedom dividend).
I think both of those approaches are more likely to work, but it's not nearly as emotionally satisfying as an answer which is a full throated condemnation of the bad racist people
(and to be clear white nationalists really are evil and bad). This second answer externalizes the problem. There aren't underlying structural reasons we have more bad racist people today than in recent decades. They are just bad people, we need to band together, fight and defeat them, and then our lives will be better.
(And to be clear, it really is important for society to come together and condemn white nationalism and other hateful and discriminatory ideologies wherever they rear their heads. I just don't think that doing only that will be enough to solve things.)The best analogy I can think of is when you hear people on the right talk about islamic extremism or islamic terrorism. Everyone agrees terrorism and violence is bad, but left-wing responses to questions about that issue tend to focus more on "how to we prevent muslims from becoming radicalized in the first place" and right wing responses to questions about that issue tend focus more on "I'm going to protect you from those bad muslims, or even from muslims generally.*"
Switch "left" and "right" for two different wings of the democratic party, and "islamic terrorism" for "white nationalism" and you have the dilemma democratic presidential candidates confront today.
*"...But the approaches I will use will radicalize other muslims who previously weren't."