How did the Democratic Party move so far left so fast? No convention, retreat or caucus was held to designate Medicare for All, tippy-top tax rates, arcane identity litmus tests, free college tuition or a Green New Deal as Democratic dogma, but here we are.

So far every major Democrat who has declared for the presidency—Elizabeth Warren, Kamala Harris, Cory Booker, Kirsten Gillibrand, Amy Klobuchar, Tulsi Gabbard—has felt obliged at least to genuflect in the direction of Cardinal Bernie.

If...