Up through the 1992 elections, the South was still solid for Democrats below the Presidential level. This was the reason why Democrats usually held a majority in the House from 1932 to 1994.
The South had long left Democrats at the Presidential level after the 1964 election, only coming back to support fellow Southerner Jimmy Carter in 1976. Even as these Southern districts were voting in some cases at almost 80% for Richard Nixon and Ronald Reagan and in a couple cases even George HW Bush, they still elected Democrats to the House.
For example, as late as 1990, Democrats held five of seven House seats in Alabama and came within a point of winning a sixth. In Georgia, they held nine of 10 seats and came within 500 votes of winning all 10. Basically all Southern districts were held by Democrats except for wealthy suburban districts(which ironically, are the ones that Democrats seem to have the best shot at winning these days outside black majority districts).
Right now, here are the Southern Delegations and the Democrat to Republican ratio of seats:
Alabama: 6R-1D
Arkansas: 3R-1D
Florida: 19R-6D
Georgia: 8R-5D
Mississippi: 3R-1D
North Carolina: 7D-6R
Oklahoma: 4R-1D
South Carolina: 5R-1D
Tennessee: 7R-2D
Texas: 23R-9D
Virginia: 8R-3D
Now lets assume that the South was still solidly Democratic except for wealthy suburban districts.
Alabama: 7D
Arkansas: 4D
Florida: 23D-2R
Georgia: 11D-2R
Mississippi: 4D
North Carolina: 12D-1R
Oklahoma: 5D
South Carolina: 6D
Tennessee: 7D-2R
Texas: 27D-5R
Virginia: 9D-2R
You can see that if Democrats still had this solid South to prop them up, Democrats would now hold 270 seats in the House, 52 above a majority.
With the solid South now solidly Republican, Democrats are going to have to look elsewhere to win control of the House. They are going to have to win back seats in Pennsylvania, Ohio, Michigan, upstate New York, California, and across the Mountain West. Florida is one Southern state where there are opportunities for Democrats to win seats, as are Texas and Georgia down the line.