On a break during a business trip to Washington last year, David Panton hailed a cab to take him to the Capitol. He told the driver he was going to see the Texas senator and presidential candidate Ted Cruz. “He’s racist,” the cabdriver replied, according to Mr. Panton. Mr. Panton, taken aback, informed his driver that Mr. Cruz had a bust of the Rev. Dr. Martin Luther King Jr. on the right side of his desk, that he was the only senator to attend the funeral of Nelson Mandela and that he had a “black guy” as a college roommate...