Last week, Rachel Shabi, a left-of-center British journalist, warned American progressives that anti-Semitism could one day tear apart Democrats just as it is now doing to the Labour Party. When I read her essay, just ten days ago, it seemed fantastical. Today, after watching progressives floundering about in the wake of Ilhan Omar’s smearing of pro-Israel activism as a form of dual loyalty, Shabi’s diagnosis looks prescient. It can happen here.

Shabi’s argument is that three conditions exist to help spread anti-Semitism among even progressives who are not inherently predisposed to it. The first is the right uses hyperbolic accusations of anti-Semitism to close off legitimate criticism of Israel. Second, the right is comprehensively more bigoted than the left. And third, the left itself is divided, so that when a member of the more radical faction is identified with it, “anti-Semitism quickly became part of an ongoing factional battle.”

All these conditions drive many leftists to form a protective cordon around their allies who promote anti-Semitic tropes. Only the most hard-core members actually defend anti-Semitic ideas on the merits. Most of them instead are driven into this position by polarization, defending anti-Semitism as an act of defiance against political enemies inside and outside the party.