Final answer:
A shift in the political allegiance of southern states from Democratic to Republican is the significant development associated with the "Republican South." This transition became evident by the late 20th century, influenced by civil rights discussions and cultural changes, culminating in the 1980 presidential election with the evangelical movement's support for Ronald Reagan.
Step-by-step explanation:
The significant political development associated with the "Republican South" refers to the shift in the political allegiance of southern states from Democratic to Republican. This realignment became pronounced by the late 20th century as a response to various social and political factors, including civil rights legislation and broader cultural shifts. During the civil rights movement of the 1960s, regional disagreements over social issues like civil rights began to fracture the Democratic bloc in the South, leading to an exodus of many white southern Democrats to the Republican Party, which was beginning to align more with conservative values and states' rights.
This transformation was largely completed during the 1980 presidential election, when the evangelical movement shifted its support from Jimmy Carter to Ronald Reagan, contributing to the rise of the Republican Party in the South.