Democrats are worried that they have a problem: The three people leading polls in the very early stages of their presidential race are all white men. The party traditionally battles over identity politics and wants to be seen as promoting diversity. Its last three nominees have been Barack Obama, who became the nation’s first African-American president, and Hillary Clinton, the first woman to win the popular vote. Vice President Joe Biden, Sen. Bernie Sanders (I-Vt.), and Rep. Beto O’Rourke (D-Texas) are this year’s top-tier candidates, according to a recent and very early Des Moines Register/CNN/Mediacom poll. It showed that 32...