Earlier this year Sen. John Kerry caused a stir by saying that the Democratic party “always makes the mistake of looking South… . Al Gore proved he could have been President of the United States without winning one Southern state, including his own.”

Kerry was lamenting the party’s perennial efforts to woo back the Southern states that once reliably stood in the Democratic column. For 70 years Republicans were effectively shut out of the “solid South,” a result of their having been the party of Lincoln, abolition, and Reconstruction. But over time, as the Democratic party emerged as a champion of black civil rights and then embraced the rights revolutions of other groups—women, gays, lesbians—white Southern voters shifted their support to the GOP.

Jimmy Carter gained the Presidency in 1976, but no other Democratic presidential candidate has won more than four Southern states; in 1972, 1984, 1988, and 2000 the Democrats lost the entire South. At the heart of this defection was not just a white backlash against civil rights but a sense that the party had embraced the social excesses of the late 1960s.

Many writers trace this rift to the disaster of 1968, when at its convention in Chicago the Democratic party simply imploded. That famously explosive week saw party regulars and antiwar insurgents trade vicious barbs while Mayor Richard Daley’s riot police—12,000 strong, augmented by 11,000 federal and National Guard troops—fought in the streets with upward of 10,000 protesters. The Democratic party entered the 1968 fall campaign badly divided and dispirited, and when Hubert Humphrey lost the November election to Richard Nixon, it was the start of a long decline. Since 1968 Democrats have lost six out of nine presidential elections.

Yet the woes of the Democratic party didn’t originate in Chicago, or even in 1968. They can be traced back to another convention, in another city, in another year. Forty years ago this summer, the Democratic party met in Atlantic City to nominate the incumbent President, Lyndon Johnson, for another term. Nobody knew it then, but that 1964 Democratic National Convention would be a turning point for the party. It was Atlantic City that sowed the seeds of the internecine wars that tore apart the Democratic coalition four years later in Chicago and that have left it wounded ever since. ...