A television ad for Hillary Clinton’s presidential campaign now airing in South Carolina shows the candidate declaring that “too many encounters with law enforcement end tragically.” She later adds: “We have to face up to the hard truth of injustice and systemic racism.”

Her Democratic presidential rival, Bernie Sanders, met with the Rev. Al Sharpton on Wednesday. Mr. Sanders then tweeted that “As President, let me be very clear that no one will fight harder to end racism and reform our broken criminal justice system than...