America is polarized in many ways, but one of the most significant is between generations in the Democratic Party. Coming out of the Great Depression and World War II, we present-day seniors saw liberalism as the promise of racial and social justice and broadly shared prosperity. We also saw it as a defender of civil liberties against abuses such as those that took place in the 1950s McCarthy era. Abroad, we supported a strong United Nations and other multilateral institutions to reduce conflict but had no illusions about the expansionist ambitions of totalitarian states.

In...