American politics has gotten steadily more partisan over the last several decades, mostly as a result of desegregation. While the south was under an apartheid regime many Republicans ("Rockefeller Republicans") were to the left of Democrats ("Dixiecrats.") This is no longer the case; every Democratic senator is to the left of every Republican senator - if you have strong politics yourself, the absolute distance looks small, but the lack of mixture is an undeniable fact. The decreased importance of regional party machines plays into this as well. Parties now function much more like coherent policy packages, so legislators have less allies outside of their own party.