"There is nothing immutable about the way the two parties currently line up"
The Economist documents the fractures within the Republican and Democratic Parties.
This Economist article is well worth reading. A snippet:
There is nothing immutable about the way the two parties currently line up. Republicans used to be the big-government progressive party, formed in opposition to slavery and pushing to remodel the South after the civil war; they have also been the small-government party, not only now, but in opposition to the New Deal in the 1930s. Democrats were once the small-government party, opposing those who wanted a more powerful federal government and defending the interests of white southerners against Washington; now they are famous as the big-government party, pushing federal anti-poverty programmes in the 20th century and government involvement in health care in the 21st.