Opinion | I’m a Presidential Historian. This Is My Biggest Regret About Trump.

I thought I knew what we were dealing with. When Donald Trump began his rise to power in 2015, he struck me as a dangerous but recognizable demagogue. As a biographer of presidents, I tend to think historically and seek analogies from the past to shed light on the present. And so, for years Mr. Trump’s marshaling of fear, prejudice, resentment, xenophobia and extremism put me in mind of grievance-driven figures ranging from Huey Long to Joseph McCarthy to George Wallace. To me, Mr. Trump was a difference not of kind (we had long contended with illiberalism in America) but of degree (since the Civil War, no figure with such illiberal views had ever actually won the White House).

Then he proved me wrong. His concerted efforts to overthrow the November 2020 election very nearly succeeded — tangible proof that he is in fact willing to follow through on the authoritarian threats he so freely makes. I now see him as a genuine aberration in our history — a man whose contempt for constitutional democracy makes him a unique threat to the nation.

I say this not as a Democrat, which I am not. I first encountered the drama of American politics through a childhood interest in Ronald Reagan, whose public grace struck a chord within me. (At 10, I was not very astute about the implications of supply-side economics.) I became the biographer of George H.W. Bush. I have voted for both Republican and Democratic nominees for president and down the ballot. And I have spent much of my adult life studying and writing about the office that John Kennedy called “the vital center of action.”

Analogies thus come naturally to me. Yet more and more, I fear that trying to find historical precedents for Mr. Trump presents dangers of its own. No similar figure in American history has ever had such a strong grip on so many. To suggest otherwise diminishes the sense of urgency the moment requires.

I wish I were overstating the case. But I am not. Given our binary system, a vote for Kamala Harris is a vote for a democratic ethos in which we can pursue lives of purpose and prosperity under the rule of law. A vote for Donald Trump puts that ethos at risk.

Democracy is fragile and human, and its success turns on how well — or how badly — Americans manage their own appetites. Nothing in the past decade suggests that a re-elected Mr. Trump would have any incentive to curb his own. That his attempted coup failed should not be grounds for dismissing the threat he poses; rather, that it was attempted at all should persuade us not to endanger the constitutional order again. And to dismiss his own radical words as well as the concerns of those who worked with him that he harbors dictatorial ambitions is to put faith in a man who has already shown himself to be more interested in himself than in the nation, more devoted to his aggrandizement than to the Constitution.

A second Trump presidency is an open invitation to chaos. A Harris presidency, on the other hand, would be a sequential chapter in the American story — a comprehensible undertaking within the vernacular of power as practiced by presidents dating back to Jefferson, Jackson and Lincoln. You may disagree with her, but a President Harris would govern in the tradition that includes Democrats and Republicans.

Differences of policy pale in context. The point of our democracy is to debate and to disagree within an arena defined by the rule of law and informed, ideally, by respect for conventions that enable us to endure without falling into a Hobbesian war of all against all. A Harris presidency would preserve that arena. Another Trump presidency could destroy it.

History cannot comfort us in this hour. But it can inspire us. To my Republican friends, my plea is straightforward: From Gettysburg to Omaha Beach to Selma, Ala., Americans have fought and bled and died so that we the people could seek to perfect our union — not so that an authoritarian showman-bully could turn our national project into his own fief.

America has always been shaped by the tension between hope and fear, justice and injustice, grace and rage. Whether the good prevails over the bad — whether we move closer to the promises of the Declaration or farther away from them — is contingent on the habits of heart and mind of a sufficient number of Americans, in power and far from it. It took a cataclysmic civil war to end slavery. It took an attack on Pearl Harbor by one Axis power and the subsequent declaration of war on us by Nazi Germany to bring America into the fight against fascism in the middle of the 20th century. And it took innumerable acts of nonviolent protest to end legalized segregation. We, in other words, never simply wake up one morning and decide to bend the arc of the universe toward justice. As the old aphorism puts it, Americans do the right thing only once we have exhausted every other possibility. Today, we have a chance to do just that: the right thing.

Jon Meacham, a historian, is a professor at Vanderbilt and the author, most recently, of “And There Was Light: Abraham Lincoln and the American Struggle.”

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