The “you ain’t black” thing is a combustion of two pretty well-known factors: CTG rattles people with direct questions, and Biden gets set off if he thinks someone is questioning his record.

I’ve seen it when Biden got challenged by climate activists or someone had a Q about guns — cue for a 60-second stemwinder about how the questioner should check the facts and “I’m the only one who’s beaten the NRA” etc

You might ask “why doesn’t he pivot to a better topic.” He doesn’t pivot! If you doubt he’s got the best positions and experience, he’ll tell you you’re wrong and should vote for someone else.

So far this has gotten him the nomination and a poll lead.