This year, retirements of Democrats have left the party scrambling to retain four open seats in Arkansas and Tennessee that have been in their control for most of the last century. Those districts, along with others held by incumbents in Alabama, Georgia, Mississippi and North Carolina, are central to the Republican strategy to win the House.

For the first time since Reconstruction, Republicans also are well-positioned to control more state legislative chambers and seats than Democrats in the South, which would have far-reaching effects for redistricting.

“It’s not a good prospect for the Democratic Party in the South,” said Glen Browder, a former Democratic congressman from Alabama. “It should be a moment of reflection for Democrats. When you forfeit the South, your sights tend to drift too far left.”…

“It’s getting easier to run as a Republican here,” Mr. Crawford said in an interview. “We have a lot of folks who have moved into Arkansas who were not necessarily brought up with the idea that they had to vote Democrat because their daddy did it and their granddaddy did it that way.”