At the 1964 Democratic convention (which was fascinating to watch on TV), the Mississippi delegation walked out because they'd been replaced by an integrated delegation. That was the beginning of the South questioning Democrats.
The change was all talk about 1965-75, then surged while that smiling Southern gentleman Reagan was President.
It's like the (heterosexual) sexual revolution. Premarital sex was mostly just talk in the 60s (novels, movies, other media), but people (radical young long hairs) actually started living together unmarried en masse in the 70s, and by the 80s it was default to live with people of the opposite sex a few years before marrying them.
Obviously there were many exceptions, but the idea is that a social change is all theory for a while, then it's all talk in the media for a decade, then radicals do it for a decade, then it becomes default and mainstream.