Kamala Harris introduced herself to the American public on Thursday, and her presentation was much like this week’s Democratic convention: well delivered, confident and optimistic, and mostly devoid of policy substance. Whether she can keep this up, unexplained and unexposed, for the next 12 weeks will determine whether she becomes America’s 47th President. The Vice President spoke with authority and admirable brevity at 37 minutes. She opened with a lengthy account of her own biography...