Let us be real, the democrat party has been promoting socialism and communism since the first civil war. Just not everyone wanted to admit it.
In the 19th century, the Democrat party was the party of limited government, decentralized power and lower taxes. They were the exact opposite of what they became by the late 20th century.
That’s why the Southern states left the Democrat party. The South’s views did not change. The Democrat party’s views changed.