JFK and LBJ supported civil rights and the end to Jim Crow. This basically flipped the southern states from Democrat to Republican.
Consider this quote from Kevin Phillips, one of Nixon's political strategists:
Quote:
From now on, the Republicans are never going to get more than 10 to 20 percent of the Negro vote and they don't need any more than that... but Republicans would be shortsighted if they weakened enforcement of the Voting Rights Act. The more Negroes who register as Democrats in the South, the sooner the Negrophobe whites will quit the Democrats and become Republicans. That's where the votes are. Without that prodding from the blacks, the whites will backslide into their old comfortable arrangement with the local Democrats.
Boyd, James (May 17, 1970). "Nixon's Southern strategy: 'It's All in the Charts'", The New York Times, pp. 215.
The success of the southern strategy led to the use of anti-gay campaign themes in more recent elections - get the people afraid that the dangerous minorities are going to destroy society, and position the Republicans as the only ones who will stand up to oppose it.