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Within Democratic President Lyndon Johnson’s landslide victory in 1964, there was a compelling regional story: many voters in the Deep South, long loyal to a conservative wing of the Democratic Party, voted for the Republican candidate.

Two years later, the 1966 midterm elections would accelerate that trend, with voters across the South electing Republicans to offices for the first time in nearly 100 years.

For generations, Southern politics had been dominated by a conservative, segregationist wing of the Democratic Party, a legacy reaching back to the Civil War, when the South sought to secede from the country to preserve slavery.

When President Johnson signed the Civil Rights Act after a bruising battle in Congress, an aide recalled Johnson privately fearing an exodus of white Southerners to the Republican Party.

“He understood that you have progress, but there will be a price to pay. There will be a backlash against that,” Sam Fulwood, a journalist and the author of “Waking from the Dream: My Life in the Black Middle Class,” told Retro Report.

It was a key moment that affects American politics today, as the two parties began to sort themselves out regionally and ideologically into two distinctly partisan camps.

“We are still living in the world of that mid-1960s political environment in many ways,” said Matthew Dallek, a political historian at George Washington University. “And we see the parties reflect aspects of the debates from that era.”

MATTHEW SPOLAR is a producer at Retro Report. This article first appeared in Retro Report’s free weekly newsletter. Subscribe and receive lessons from history in your mailbox. Follow us on Twitter @RetroReport.