In a 2016 book, “Insecure Majorities,” the University of Maryland political scientist Frances E. Lee argues that this narrow split between the parties does as much as ideology to explain today’s tribal rancor. When the Republican Party seemed consigned to the role of a hapless minority, it had little incentive for scorched-earth partisanship. In the 1980s, though, every election came to offer the tantalizing possibility of power changing hands. In Congress, partisan activism began to supplant policymaking. Tactical obstructionism by the minority and “partisan message votes” — called by the majority purely to show voters a distinction between the parties — became a large part of what lawmakers did. Increasingly, they hobbled Congress’s ability to do anything else.

“Partisan,” as an epithet, bemoans this fall from an Eisenhower-era, country-before-party grace. But really, how many liberals would choose that over the expanded rights and protections of the ’60s and ’70s? How many conservatives would volunteer to pay midcentury tax rates? The problem is that Americans live amid two stalled partisan revolutions, held in stalemate by demographic parity.

It’s true that the public’s dissatisfaction with partisan fighting has increased over the course of this stalemate. But according to the American National Election Studies, a survey conducted regularly since the late ’70s, our willingness to take a side in those fights, and identify with one party, has barely slipped over the past 40 years. What has shifted — significantly — is people’s dissatisfaction with the other side. The ANES survey asks subjects to rate both parties on a scale of zero to 100, with 50 being neutral. Over the years, the ratings that partisans have given their own parties have not changed much, but the ratings they give the opposing party have plunged, from around 50 in the Carter years to less than 30 during the Obama administration. More recently, the percentage with extremely negative views of the opposing party has climbed steeply, to about 50 in 2016 from about 20 in 2000.

In this context, the American reflex against open partisanship might be best understood not as a real belief but as a sort of taboo — a lingering remnant of an old political culture. Donald Trump, of course, exists to shatter taboos. Instead of solemnly interrupting his midterm campaigning schedule after the Louisville and Pittsburgh shootings, he continued; hours after Sayoc’s arrest, Trump regaled a crowd in Charlotte, N.C., with the same demonology of Democrats that Sayoc embraced. This prompted Max Boot, a conservative writer turned Republican apostate, to accuse Trump, in a Washington Post column, of “risking widespread political violence so that he and his Republican supporters can hold on to office.” In a time of crisis, he wrote, Trump was “deliberately exacerbating our divisions for partisan gain.”

But does Trump represent a shift in kind from his predecessors or merely an escalation of shamelessness? Forty-three years ago, Rusher sought to leverage civil rights and identity politics to sever “businessmen, manufacturers, hard-hats, blue-collar workers and farmers” from the Democratic Party — a project he framed as necessary to make the Republican Party stand for something bigger and more enduring than the sum of its individual politicians. Funny that the benefits of his success have accrued to a politician who stands for little more than himself.