The scene played out dramatically at the Georgia Republican convention where Goldwater spoke. He left for California immediately after his speech and thus missed the political decimation of Georgia’s Eisenhower Republicans. In a six-hour political rout, hard-line segregationists swept them out, along with longtime African-American leaders. “What has been done here is to read the Negroes out of the Republican Party in Georgia,” complained one high-ranking white official.

The new order was symbolized perfectly later that campaign season when South Carolina’s Democratic senator Strom Thurmond, the Dixiecrat presidential candidate in 1948, switched parties. Thus was established the political strategy in the South that Republican presidential candidates have followed ever since — melding an overtly conservative, socially moderate economic appeal aimed at the middle class with a politics of rage geared toward disaffected white voters.

Richard M. Nixon pulled it off artfully in his two successful campaigns, appearing mostly in Southern cities and suburbs and letting Thurmond work the Deep South circuit. Ronald Reagan folded in religious conservatives in the 1980s to replace the generation of Dixiecrats dying off, thus consolidating the powerful mix of cultural reaction and economic conservatism that is modern Republicanism.

Yet this year that mixture may not work. Mr. Trump’s extreme language and divisive policies are alienating moderate Republicans in places like the Atlanta exurbs — where Mrs. Clinton is running nearly even with Mr. Trump. And across the state, polls show a significantly low number of Republicans saying they’ll support their party’s candidate.

Mr. Trump’s campaign most closely resembles the presidential campaigns of George C. Wallace, the arch-segregationist Alabama governor. Indeed, Wallace’s legacy is telling. An economic progressive, he remained a Democrat his entire life. True, he galvanized white working-class disenchantment and pioneered a populist, anti-liberal rhetoric that Ronald Reagan and subsequent Republicans would use to devastating effect. Yet he never had much appeal among the new class of suburban whites; the two were like oil and water. So, too, it would seem, are Donald Trump and moderate Southern Republicans today.

Whether or not Republicans hold on to Georgia and South Carolina this year, the lessons they are likely to take away are predictable. Democrats will assume that these states, like Virginia and North Carolina, are part of a long-term liberal trend and push traditional liberal ideas harder in future elections. Republicans will most likely write off Mr. Trump as a one-time phenomenon and not do anything. In doing so, both parties will ignore lessons from the history of the Southern conservative majority.

What might be happening instead is something new in the South: true two-party politics, in which an urban liberal-moderate Democratic Party fights for votes in the increasingly multiethnic metropolitan South against an increasingly rural, nationalistic Republican Party. If that happens, it will transform not only the politics of the American South, but those of America itself.