It would go something like this: I would be chatting with a seemingly nice person who would complain that a brother-in-law had lost a job. As I sympathetically listened, there would be a brief, unrelated mention of a black man who was hired for some other job. Just as I was squinting to try to comprehend the point, a vile and thunderous racist rant would be unleashed.

I ran back to my classmates who were born in this country, in horror, wondering what had happened.

“Oh, you don’t know the code,” they told me with a laugh.

“The code” was their shorthand for how racists sent out feelers to find kindred spirits. Since many people of all races opposed racism, racial identity itself was no guarantee of agreement. I didn’t know the markers of this “code,” so I sometimes failed to recognize them, or responded inadequately to them.

Today, this feeling-out process happens online and is much quicker, resulting in cascading self-affirmation. People naturally thrive by finding like-minded others, and I watch as Trump supporters affirm one another in their belief that white America is being sold out by secretly Muslim lawmakers, and that every unpleasant claim about Donald Trump is a fabrication by a cabal that includes the Republican leadership and the mass media. I watch as their networks expand, and as followers find one another as they voice ever more extreme opinions.

After many months of observing Mr. Trump’s supporters online, I wanted to see this phenomenon in person, so this month I attended a Trump rally in Fayetteville, N.C.

I tried a few conversations that sought to challenge the attendees’ beliefs, but they went nowhere for a simple reason: His supporters and I did not share the same factual universe. At one point, I heard Mr. Trump declare that Congress had funded the Islamic State. I looked around, bewildered, as there was no reaction from the crowd. My social media forays confirm that even that was not an uncommon belief.

Mr. Trump doesn’t only speak outrageous falsehoods; he also voices truths outside the Overton window that have been largely ignored, especially by Republican elites. For example, academic research shows that rather than deep cuts, Tea Party voters actually favor government programs, as long as they perceive a benefit for themselves. It’s fairly obvious that the current model of global trade provides a lot more benefits to corporations than to workers, and yet it took Mr. Trump’s rise to have this basic issue widely covered. In Fayetteville, Mr. Trump complained that much of the military’s expensive weaponry had been purchased simply because the large corporations selling it had political clout. As he said this, the people around me, many of them from military families, leapt to their feet in approval.