Complex sentence: Southern white Democrats comprised a powerful voting bloc in Congress until the mid- 20th century and their representatives, re-elected repeatedly by one-party states, became senior members, controlling numerous chairmanships of important committees in both houses.
Split into simpler sentences: Southern white Democrats comprised a powerful voting bloc in Congress until the mid- 20th century. Their representatives, re-elected repeatedly by one-party states, exercised the power of seniority, controlling numerous chairmanships of important committees in both houses.