People who don’t join political parties imagine that membership is an expression of opinions held in common. It starts that way, but over time, party loyalty comes to be defined at the threshold of tolerable extremism. What ugly attitude can you rub along with without recoil because, politically speaking, it’s family? That is the question that Lee Anderson, a former deputy chair of the Conservative party, forced on fellow Tories with his assertion that “Islamists” have “got control” of Sadiq Khan, the mayor of London. The whip was withdrawn. Rishi Sunak saw a line being crossed but struggled to name...