A few days before President Trump’s Twitter rant against four congresswomen of color, a few thousand progressives gathered in Philadelphia to hear speakers exhort them to make race a central issue in the 2020 election. “We’ve got to center racial justice,” Rashad Robinson, the executive director of Color of Change, told the Netroots Nation conference. Robinson told the audience that “racial justice is a strategy for gaining power and winning power.” Robinson represents a left wing of the party that wants to confront Trump’s demagoguery on race and identity head on, not shying away from the topic. “Democrats for years...