SINCE Donald J. Trump shot to the top of Republican polls last fall, pundits have tried to make sense of his popularity. He has been described as a modern-day product of reality-TV narcissism, or the second coming of European fascism. But as he cruises into the South Carolina primary after beating his rivals by double digits in New Hampshire, it’s clear that neither idea quite explains his strength.

Mr. Trump’s rhetoric resonates with a particular American political tradition. Voters may not know the details of that tradition, but they feel it viscerally when a politician taps into it. Mr. Trump has done just that by emulating a classic model of American democratic leadership.

A clue as to just which leadership model can be found on a map. While Trump fans are spread across the country, they are heavily concentrated in and near the Appalachian states — from Mississippi and Alabama all the way to western Pennsylvania and New York. The northwest corner of South Carolina is one of the most pro-Trump parts of the country.

Greater Appalachia has remained culturally distinct for centuries. Migrants from the northern British Isles — Scots, Scots-Irish and others — pushed into these mountains in large numbers from the 1700s onward and did much to create the nation as we know it. Their descendants weathered generations of hardship and calamity: washed-out hillside farms, coal-mining disasters and extreme poverty.