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These are Democratic primaries. His success so far shows that he can win white Democratic votes. If that was ever in doubt, we sure are not the party we thought we were.
Only the general election will show if he can win white votes across the board in a country that is still very racist in my view. The Republicans have used the Southern Strategy to great effect in recent elections - we'll see if it still works. This is the best chance to test it, since their field of candidates is so weak and their president so unpopular.