“If I pack up my toys and go home, there are people in red MAGA hats who would be saying, ‘Don’t let the door hit you on your way out.’” Kristen Soltis Anderson is losing faith in her party. And that should trigger alarms for Republican leaders concerned about the GOP’s long-term health. Anderson is a smart and telegenic young Republican pollster. She has specialized in studying how the party can improve its anemic performance among the Millennial generation, which will pass the right-leaning baby boomers to become the largest generation of eligible voters in 2018. Now she is wondering...