When the Democratic Party faced a revolt from its ranks for daring to propose condemning anti-Semitism, the scene gave those of us in Britain deja vu. The American Left is following the same script that led to the rapid radicalization of the Labour Party, led by Jeremy Corbyn. It’s no longer a mere talking point, a form of shorthand for journalists: The Democratic Party’s Corbynization is here, and it tacks so closely to what happened in Britain that it’s important for Americans to understand where we’ve been — and where they’re headed. The anti-Semitism fracas came about thanks to a series of bigoted statements made by Minnesota freshman Democratic Rep. Ilhan Omar, who accused U.S. Jews of disloyalty to America. Omar was part of the January intake in the House of Representatives, alongside Rashida Tlaib of Michigan and Alexandria Ocasio-Cortez of New York. The trio have become increasingly dominant in press and social media coverage, allowing them considerable influence in setting the agenda for the party in the public space. Tlaib has also levied the “dual loyalty” charge, and Ocasio-Cortez has used her prominence to defend them. The Democrats regarded this as opening the way for a bold, new approach to politics, with fresh ideas from a younger cadre who would broaden the debate. I know where I’ve heard that before. In September 2015, Corbyn became the leader of the British Labour Party. There were...