To Donald Trump supporters everywhere: We have work to do. Real work. A lot of work. All of us.Since last summer, it looked like Donald Trump’s momentum and power was unstoppable. It was easy to cheer him on, watch him rack up victory after victory, and get excited for a likely nomination and balls-out battle against Hillary. We knew the backlash was fierce and the opponents were minion, but they were losing the arguments, and Mr. Trump’s numbers just kept rising. It was a really fun and joyful experience. Today, it’s different. Mr. Trump is a magnificent campaigner, but he...