Many pre-Nixon Democrats were souhteners who still retained vestigaes of the Old South - racial prejudice and fervent nationalism with isolationist tendencies. Post Nixon, they became Republicans. That left the Democratic Party with a core of left of center members.
You have a very valid point. Yesterday's Dixiecrats are today's Republican Party. Though they may have been isolationist pre-Nixon, I think they've taken an opposite turn after his presidency.
What's really interesting is to see how the democratic party has evolved. It's evolved from the early 60's from the New Frontier set out by Kennedy, and took on to such a counter-culture after he was assassinated that his supporters took a fascination to those he opposed (Castro, other communists..). It's outlined in this book I got for Christmas, which I would recommend to anyone who's interested in the political thought current flowing in this country during the 1960's. It's called
Camelot and the Cultural Revolution: How the Assassination of John F. Kennedy Shattered American Liberalism. Kennedy has captured my interest very much, not only because of the highly suspicious assassination, but also because of his short presidency, and the ideas to come from it.
