At this stage in a presidential campaign, Republicans, generally a rather disciplined lot, have usually united and begun to train their fire on Democrats. Circular firing squads are for them, not us.

Not this year.

The candidacy of Donald J. Trump is not only fracturing the Republican Party, it is breaking up friendships as well.

A prominent Republican, describing a Trump-related disagreement with another influential Republican with whom he has been close for decades, sent me a note that stated things in a matter-of-fact way: “We had a friendship-ending email exchange.”

Others have confided that differences over the Trump candidacy have caused such a loss of respect that they feared their friendships would not survive, and that even if they did, they would never be the same.

While I haven’t lost any friendships during this Trumpian moment, at least not yet, I certainly haven’t been immune to the heightened tension. Several friends whose political views have often coincided with mine in the past have voiced their anger to me over my public opposition to Mr. Trump’s candidacy.

One close longtime friend told me that my criticism of Mr. Trump stemmed from my desire for attention and notoriety and a longing for the favor of liberals. He was questioning not my reasoning but my motivations. His concern wasn’t about policy; it was about the state of my soul.

Last week, a friend I am in frequent contact with and who is sympathetic to Mr. Trump informed me that my attitude was “unhinged” and utterly close-minded. A woman I attended church with for several years expressed her unhappiness with my anti-Trump “screeds.”

These people aren’t stupid or malicious; they are upset because I see things in a profoundly different way from them and because I have referred to a candidate they like as the avatar of unreason.

Strained relationships resulting from political differences are pretty common. What makes this moment so unusual is that the ruptures are occurring among people who have for years been political allies, whose friendships were forged through common battles, often standing shoulder to shoulder.

This dynamic is playing out in public, too. Glenn Beck and Sarah Palin, while not lifelong friends, were once close. Mr. Beck described her in 2010 as one of the few people who could “possibly lead us out of where we are”; Ms. Palin referred to him as an “inspiring patriot.” Yet in part because Mr. Beck supports Ted Cruz while Ms. Palin supports Mr. Trump, they now trade insults. Mr. Beck accused Ms. Palin of abandoning her principles, while she has mocked Mr. Beck for having distributed care packages to illegal immigrant children.

Similar stories are happening all over. National Review’s Jonah Goldberg, whose opposition to Mr. Trump has also put him at odds with people he has long liked and respected, admitted, “I hate the idea that political disagreements will poison friendships.”

The reason for the envenoming is Mr. Trump, who inspires deep loyalty among his followers and revulsion among his critics. For some, he is a breath of fresh air: perhaps a bit rough around the edges, but a strong person, plain-spoken and able to make America great again. Others, like me, consider him emotionally unstable, unprincipled, cruel and careless, the kind of demagogic figure the ancient Greeks and the American founders feared.

Given the fundamental and intense disagreement over the advent of Mr. Trump, then, we should not be surprised that even longtime friendships are feeling the strain. In his short book “The Four Loves,” C. S. Lewis writes that while lovers stand face to face, friends stand side by side, absorbed in common interests, seeing some common truths. When these common truths become competing truths, a distancing is inevitable — perhaps especially when political differences arise among people who have devoted their lives to politics, who view it as a means to advance justice and human flourishing and therefore consider it a core part of who they are.

And therein lies the problem: When political differences shatter friendships, when we attribute disagreements to deep character flaws, it usually means politics has become too central to our lives.

I will be the first to admit that for those of us who inhabit the world of politics, political differences aren’t trivial. I am guilty of having sometimes lost sight of the fact that friendships aren’t meant to reinforce every one of my views. But the best friendships are those in which one person elevates the sensibilities of the other, including from time to time helping us see things from a different angle. They are, as Aristotle put it, friendships of virtue rather than of utility or pleasure.

Years ago I wrote my friend and mentor, Steve Hayner, worried that our differences over a political issue we both had strong feelings about might hurt our relationship. Our relationship mattered more to me than politics, I told him, and I didn’t want a breach to occur.

“I want to assure you that I don’t think that our disagreements on most anything could affect our relationship,” he wrote to me. “My love for you has nothing to do with your views.”

My relationship with Steve, who died last year, was among the deepest in my life. I had known him since college — I was a student and he was an associate pastor — and he had accompanied me through times of joy and hardship. This insulated our relationship against mere political differences. But the main point applies even to friendships that may have taken root in the soil of politics: We need to work to stay in relationships with people despite deep differences of opinion, not just across the aisle but on either side of it.

This isn’t always easy. One example: I was friends with a journalist with whom I had some similar instincts, if not complete political agreement. Our relationship was characterized by respect, affection and interests beyond politics. Yet after I joined the Bush White House in 2001, it hit a very rough patch. I sensed that he believed I had gone over to the Dark Side, loyally supporting indefensible policies; I felt that he was unfair and unreasonable in his critiques of our administration. Neither of us was inclined to give ground; each of us was happy to point out the flaws we suddenly saw so clearly in each other.

Thankfully, since then we have reconciled, although it took almost the entire length of the Obama administration. I felt last summer was time to explore the possibility of re-connecting. It turned out it was.

Time and distance helped repair the breach. Passions cool, the gaps between you don’t seem quite as wide. The qualities that once attracted you to others come back into focus. Conversations turn to topics deeper and more personal than politics. But the restoration of fractured friendships doesn’t happen by accident; it is a matter of choice.

Mr. Trump’s candidacy is putting more stress on more friendships than any other political development in my experience. Precisely because of the antipathy I have for Mr. Trump, I need to try doubly hard to resist the temptation to assume the worst of his supporters even as my worries about him mount. Absent compelling evidence to the contrary, I need to grant to them the same good faith I hope others would grant to me.

In his first inaugural address, with the Civil War looming, Abraham Lincoln told his fellow citizens that we are not enemies but friends. “Though passion may have strained, it must not break our bonds of affection,” he said. During his second inaugural, at the war’s end, he asked us to “bind up the nation’s wounds,” with “malice toward none, with charity for all.” This was an almost superhuman ideal, but it needed to be stated.

None of us is Lincoln, and our divisions today obviously pale in comparison to those he and the country faced. Yet we can still learn from him. Having lived through the previous decade of tumult and political division, he knew the importance an attitude of conciliation can play in the life of a nation. We should strive for a bit of the grace and largeness of spirit he showed.

Peter Wehner, a senior fellow at the Ethics and Public Policy Center, served in the last three Republican administrations and is a contributing opinion writer.