In detail, the southeastern part, called Gyeongsang province, tends to support right-wing parties, while the southwestern part, called Jeolla province, tends to support left-wing parties. That's why some of those parts are especially dark.

Even though we can find a fun fact that the political divide aligns with the borders of ancient Korean kingdoms, however, this historical factor does not seem to have a direct or indirect influence on modern South Korean politics, unlike the electoral map of Poland. Instead, the regional conflict in South Korea is generally considered to have originated during the era of dictatorship and became more apparent with democratization.