Bill Clinton being the first black president was a joke about his appearance on the Arsenio Hall show where he played the saxophone. No one actually believed that.
It wasn't merely a joke. His background growing up poor with a single mom in the rural South made him quite a bit more relatable than anyone who preceded him. It was tongue in cheek, but there were real sentiments within.
Similarly, Beto has a cultural background and upbringing that is relatable for this community in a way no other candidate preceding could claim.
