But the harsh divisions among Americans in 1968 have largely endured. They are rooted in profound disagreements based on culture and creeds that are impervious to compromise. If one thinks abortion is murder or that L.G.B.T.Q. people deserve every right that heterosexuals have, the very idea of finding a middle ground is abhorrent. The mutual hostility between religious conservatives and liberals — and nonbelievers — that emerged in the 1960s also fuels these seemingly irreconcilable differences. Each side is convinced it represents a majority — and a moral one at that.

In addition, the alienation of rural white Americans from cosmopolitan urban dwellers has only increased since big cities became entry points of immigrants from all over the world. The family reunifications made possible by the Immigration and Nationality Act of 1965 helped create today’s polyglot big cities.

The absence of a stable partisan majority also keeps our domestic conflicts on a persistent burn. Richard Nixon’s victory in 1968 and his landslide re-election four years later tore apart the New Deal coalition that had dominated national politics, with barely a pause, since the early 1930s. Of the 11 presidential contests from 1976 to 2016, Republicans have won six and Democrats five. But on just four occasions has the Republican victor gained a plurality of the popular vote.