It turns out that being white, highly educated, urban-dwelling, and older all correlate with political intolerance. The authors think this might be because such people are best able to segregate themselves into like-minded bubbles where they may never encounter someone who represents a different political tradition. Less privileged Americans, on the other hand, “have more diverse social networks, politically speaking, and therefore tend to have more complicated views of the other side, whatever side that may be.”
Of course, it’s very important to note that political intolerance is not the same thing as, say, racial intolerance. Race is an immutable characteristic, whereas membership in a political party is voluntary. We do not choose our skin color, but we can choose our political beliefs (at least in theory; The Atlantic piece notes that the “vast majority of people” stick with whatever party their parents chose for them). If someone has particularly awful political views, it’s not necessarily wrong to show them intolerance.
But if wide swaths of the population routinely refuse to engage with anyone who occupies a different position on the political spectrum, they will probably be more ignorant about what those people actually believe. They will tend to demagogue each other, and assume the worst. It’s very easy to find examples of this: Relatively privileged, elite media folks were among the most eager to wrongly assume that a bunch of MAGA-hat-wearing teenagers were harassing a Native American man on the steps of the Lincoln Memorial.