About 2/3 (67 out of 100 if i not erred somewhere) are in Texas. Not bad))))
Wallace had done relatively poorly in Texas as compared to other Southern states, and Humphrey benefited from LBJ who was president at the time. Texas has lots of counties, so there are plenty of counties to differentiate between traditional Democratic support in the rural counties, and more liberal support in Urban areas. After the 1968 election, the House breakdown was 142D:8R. By 1972 it had improved to 133D:17R.
Texas also strongly supports the military, which is highlighted in Coryell, Bell, and Lampasas around Fort Hood (the other high military presences are in urban counties such as Bexar and El Paso, where the military is less dominant).
Travis, Harris, and Dallas only swung 20%, 21%, and 29% to Nixon while the swing in Coryell was 82%.