When the party was hijacked by the moral relativists, feminists, anti-religion activists, and other leftist interest groups, some people with strong moral value started to abandon them.
"At the time, they appealed to the social-justice feelings, but held the Judeo-Christianity moral views. So, the battle was mostly in 'economic arena'."
That was the Hubert Humphrey era. By the end of the 60's,
the Dems had taken up with the hippies and had adopted hedonism, anti-establishment leanings, (including organized religion). By the early 70's, the left had completely consumed the Democratic party, and has controlled it ever since.