... Like journalists, pundits, and party leaders of all stripes, most historians have assumed that the most self-evidently shameful parts of our history were just that—“history” in the vernacular sense, meaning “in the past.” Consider a few examples. Well into the last century, leading Southern Democrats, governors and senators, publicly endorsed lynching as a “higher law” solution to the problem of black men’s appetite for white women. At mid-point in that century, around when our students’ grandparents were born, a liberal President ordered the grossly unconstitutional deportation to concentration camps of up to 120,000 peaceful Japanese inhabitants, the large majority of them citizens, on the basis of their ethnicity.

In both cases, we teach from a distance, in a cautionary way, with an implicit assurance that these abominations could not, should not, and will not happen again. Similarly, we teach outright nativism—whether the Know-Nothings of the 1850s or the immigration acts of the 1920s barring Italians, Eastern Europeans, nearly all Jews, Asians, and Africans from entering the country—as something we have overcome. Not anymore! Mr. Trump does not regard such policies as consigned to the dustbin of history. If the word has any meaning, he is proudly a nativist. He hasn’t (yet) endorsed lynching or mob violence, but he has encouraged extralegal assaults on people exercising free speech, and has been slow to reject support from white supremacists like David Duke. His proposals to bar immigration on the basis of religion, and apply special standards of policing and surveillance to citizens, who practice a particular religion, are eerily reminiscent of measures targeting Asians from the 1880s to the 1940s.

These challenges to teachers of U.S. history are specific to this fall, but have been accumulating over our students’ lives in terms of racialized polarization, lurid conspiracy theories, and anti-government posturing. They have no memory of the ordinary bipartisanship that characterized American politics until recently, with Congressional Quarterly using two categories for roll-calls, denominating members as “Northern” or “Southern” Democrats, or that even in the 1990s anyone lobbying on human and civil rights issues could rely on a core of liberal Republicans in both houses. The only presidencies they know have been intensely polarizing, beginning with George W. Bush, and escalating since 2008. When a cautious centrist like Barack Obama (center-right by global standards) is routinely caricatured as a radical “socialist,” either because of venal opportunism or a profoundly anti-intellectual and anti-historical ideological framework, something has gone awry that will not soon be fixed.

Fortunately I’m not teaching contemporary politics but a survey beginning in the 1860s. Of course, I could stop the survey at some temporal point like 2000, relying on Hegel’s familiar truism about “the owl of Minerva only flying at dusk.” But the quadrennial presidential competition is extraordinarily valuable in illuminating long-term trends and themes, making the past present, as it is supposed to be. To pretend that this starkly unusual race, addressing the largest questions of who is or is not an American, is irrelevant seems irresponsible. But how to raise these issues without alienating my students who are either Trump voters, or the children and friends of his backers? I know from occasional but painful experience that any suggestion of “we all agree” or “we all know” in the classroom setting is an unwarranted assertion of authority, deeply resented by those made to feel their views are illicit or contemptible.

Donald Trump’s candidacy has forced me to confront the implicit Whiggishness of my own teaching. The “can’t happen here” has happened. Habits of mind and speech which I thought we had surmounted (or successfully repressed) have returned in full force. There is no upward ascent “from Seneca Falls to Selma to Stonewall,” in President Obama’s evocative phrase at his second inaugural. Certainly, the unending series of police or vigilante killings of unarmed black people since Trayvon Martin’s murder in 2012 has contributed to this finally realistic assessment of our American present. There is no longer any reason to think that anything is off the table, including open discussions of internment or denaturalization based on ethnicity, public violence against targeted “enemies,” whether we call it lynching or not, or state repression along the lines of the McCarthyite committees and blacklists. Trump’s advent puts all those options back on the table, and they are unlikely to go away, even if he does.

So what to do? My recourse is to move far away from the traditional linear schema of the U.S. survey. ...