Complex sentence: By the late 1870s, white Democrats had regained their political power in the state legislature and passed measures to disfranchise the blacks, which essentially lasted until after the passage of civil rights legislation in the mid- 1960s.
Split into simpler sentences: By the late 1870s, white Democrats had regained their political power in the state legislature. This exclusion from politics lasted for most black residents until after the passage of civil rights legislation in the mid- 1960s.