Jennifer asked me a few weeks ago how the current national political system had evolved. I explained my understanding of this process by concentrating on the movement of conservative Democrats in the South into the Republican party after World War II. Opposition to civil rights and perceived threats to state's rights propelled these "Dixiecrats" to change their party affiliation from the late 1940s to the late 1960s. Strom Thurmond's unsuccessful 1948 presidential run as part of the State's Rights party was an expression of racist and socially conservative sentiments* centered in the South; Thurmond's efforts to craft Richard Nixon's "Southern Strategy" helped the latter win the presidency in 1968 [Source]. This realignment helped bring into the Republican party some core constituencies that we find currently, such as the religious fundamentalists and social conservatives with varying shades of implicit and explicit racism.
Over at Edge of the American West, Eric has recently written an excellent post about how the ideologies of our present political parties evolved. His post (and the comment thread) is much more illuminating and thorough than my cursory overview.** Yet another reason to love EotAW and benefit from the hard work of others!
* Though often correlated, I'm not insinuating there that these two things are always the same thing.
** Which is just one reason why, I suppose, he gets the awards and earns the big bucks.