Every so often, the value of liberty must be relearned and such a time may be upon America.

In 1750, my fifth great-grandfather and great-grandmother, Archibald McMullin and Martha Richards, departed Ireland and braved the Atlantic Ocean in search of liberty and opportunity in America. Martha passed away a few years later in Massachusetts, leaving Archibald alone with three sons until he remarried Anna Powell, with whom he had six more children, including Archibald Jr. By 1778, the younger namesake had enlisted in the Continental Army and would serve two nine-month tours in the Revolutionary War.

Their respective struggles forced both McMullin generations and their contemporaries to answer a fundamental question: What is the value of liberty?

Since then, history has required other generations to answer the same question. For some, the Civil War provided the circumstances. For others, it was World War II, the Cold War, or the civil rights movement. For these generations, the value of liberty was intimately felt through threats to its survival. Being an American meant defending it.

The Founders warned of the fragility of liberty and our system of government, but most Americans today have seen little evidence of that. For the past several decades, our basic rights and the integrity of our democratic system that protects them have been relatively secure. For many, they’ve seemed even automatic.

That may explain why only about 30 percent and 40 percent of Americans born in the 1980s and 1970s, respectively, believe it’s essential to live in a democracy, according to research by Harvard lecturer Yascha Mounk; while about 74 percent of those born in the 1930s consider it to be.

Perhaps the value of liberty is a lesson we must learn for ourselves, not just from textbooks or popular political discourse.

In recent decades, we’ve focused almost entirely on less critical, though still important, issues of partisan disagreement. This, along with other factors such as increasingly fragmented and politically targeted media, homogeneous congressional districts, and centralized power in Washington, have created hyper partisanship in American politics, something I also saw in other nations while serving with the Central Intelligence Agency.

Extreme partisanship is more dangerous than most people realize. It leaves nations vulnerable to authoritarianism whether in the Middle East, Asia, or America. Autocrats commonly exploit—even foment—animosity between political groups, races, and religions because it helps them to consolidate power. When groups despise each other, they’re more likely to believe the falsehoods despots disseminate about their perceived enemies.

In these schemes, the authoritarian becomes a trusted source of information for favored groups, even as he undermines their grasp on truth, and increases his control over them. As divisions widen, people become less able to find common ground on their own, leaving it to the authoritarian to make unilateral decisions, often in violation of people’s basic rights.

As we anticipate the presidency of Donald J. Trump, who has demonstrated authoritarian tendencies, and what it might mean for our freedoms, I’ve noticed among my own support base that some Americans from both sides of the traditional political spectrum are finding common ground on core American ideals. These include the truth that we are all created equal and share an inalienable right to liberty, and the importance of protecting our democracy—especially under Russian assault. This is an encouraging development.

Unfortunately, I’ve also observed committed partisans on the right and the left—even those who share concerns about Mr. Trump—attacking their own and the other side for finding common cause. They attack not the merits of substance, but out of pure partisan animus. It’s as though some Americans find intrinsic value in division and learned little from Mr. Trump’s exploitation of fissures in American society.

These are political habits many of us have learned over many years, but rote, extreme partisanship places party before country and leaves our nation vulnerable to authoritarianism.

While healthy policy differences between the traditional right and the left will continue, they should not prevent Americans from uniting in the defense of democracy and our Constitution. There are deep differences and misunderstandings between both sides, each of which have somewhat different definitions for even words like liberty and equality. But on their most basic meaning, I believe there is broad consensus. We should celebrate this; it’s critical that we do.

Mr. Trump’s tenure may provide America an important learning opportunity. Perhaps liberals will be more sympathetic to conservatives’ warnings about the danger of concentrating too much power in the federal government. Similarly, conservatives may now understand progressives’ desires to foster a more inclusive culture in which people of all backgrounds are respected. And, most importantly, we may all learn the value of liberty.

One of the greatest defenses against authoritarianism is a unified people. In the years ahead, let us embrace our common ground for its common defense.