‘We are trying to be reasonable,” an organizer for Bernie Sanders’s Seattle rally said. The black female protesters who stormed the stage became enraged. “We aren’t reasonable!” they shouted back. “If you do not listen to [us], your event will be shut down,” one of them declared to the crowd. Sanders caved to the protesters from the Black Lives Matter movement and gave activist Marissa Johnson the microphone. “I was going to tell Bernie how racist this city is, even with all of these progressives, but you’ve already done that for me. Thank you,” she said, apparently in response to...