His appeal in historically Democratic areas is a reflection of strength among new Republicans — whether they be white Southerners or white Roman Catholics and working-class voters in the North who would have had no place in the Republican Party a half-century ago.

Mr. Trump’s strength among those voters, who decades ago represented the base of the Democratic Party, helps explain the resilience of his candidacy. It’s no surprise that they are not offended by his unorthodox policy views, like his embrace of entitlement programs or his opposition to free trade. They may have moved to the Republican side, but they still have moderate views on economics.

There is evidence both anecdotal and statistical that racism was another factor in the shift of some of these voters to the Republican Party. And that helps explain why he’s withstood controversy after controversy over racially charged remarks.

Mr. Trump’s success is the culmination of electoral changes that have erased and even reversed the political divides of the post-Civil War and New Deal eras. Today, Republicans draw more than two-thirds of white Southern voters, and nearly three-quarters of white Southerners outside Florida and Virginia. They win nearly two-thirds of white voters without a college degree. They even win white Catholics in the Northeast and Midwest.