Open this photo in gallery: U.S. Vice President Kamala Harris speaks after visiting a vaccine mobilization event at the Phillis Wheatley Community Center in Greenville, South Carolina, on June 14.MANDEL NGAN/Getty Images After weeks of speculation, Vice President Kamala Harris, endorsed by President Joe Biden to succeed him as the Democratic presidential candidate this fall, will have a bench of new-generation party leaders to turn to when she moves toward the first critical decision of her candidacy: picking a running mate. By now, the list of possibilities is fairly well known: Democrats who had already seemed like potential presidential contenders...