This has been happening for a century but really took of after the war when the Democrats were seen as the more progressive party. In the 1970s and 1980s the Republicans emerged as a truly conservative party.
During the 1940s and 1950s the Republicans hounded alleged communists and put them on trial. Some went to jail, but all they had done was write or make a movie. A decade later the Democrats helped to bring down segregation.
And without meaning to offend southerners, the south, the last bastion of both slavery and segregation slowly turned from Democrat to Republican.
And I haven't mentioned Richard Nixon yet. The less said the better.
A hundred or so years ago Republicans were the ones with the progressive ideas and the urge to fight oppression. Now I think that Abraham Lincoln (who was not the party's founder but one of its earliest and certain its greatest member) would be embarassed to be a part of it.
You can read all this and more on:
http://en.wikipedia.org/wiki/History...publican_Party