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 e ntry 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.

Control of one or both houses of Congress has swung back and forth too. In 1951, the eminent political scientist Samuel Lubell observed that the Democrats were like the sun, the Republicans the moon. “It is within the majority party that the issues of any particular period are fought out,” he explained, “while the minority party shines in reflected radiance of the heat thus generated.” But for the past half-century, the major parties have been more akin to asteroids that occasionally collide but whose gyrations move neither of them much closer to the center of the political solar system.

In many ways, the civil war fought out in Mayor Daley’s city that summer has never really ended.

Since the late 1960s, officeholders have sought to gain an advantage for their side by stoking the fires of conflict. This has been a consistent habit more on the right than among liberals. “Dividing the American people has been my main contribution to the national political scene,” acknowledged Spiro Agnew, who served as Nixon’s vice president until he resigned in disgrace in 1973. “I not only plead guilty to this charge, but I am somewhat flattered by it.” During his first term in Congress a few years later, Newt Gingrich accused Speaker of the House Tip O’Neill of placing “Communist propaganda” in his office. Last June, Eric Trump emulated these predecessors as well as his own father when he told Sean Hannity on Fox that Washington Democrats are “not even people.”