How worried should Democrats be about the Democratic Socialists of America? As someone who knows the DSA inside out, I can say with authority: They should be very worried indeed. I was a DSA member for years and served in local leadership. But today’s DSA — illiberal, dogmatic and hostile to traditional American political norms — is not the organization I once knew. It includes disciplined, radicalized networks that have methodically expanded their power over the last decade in pursuit of extremist goals. As the Democratic Party grapples with the DSA’s growing influence and extremism, it would do well to...