Complex sentence: This was a major reason why Virginia voted Republican in all but one presidential election from 1948 to 2004--the one exception being 1964, while conservative Democrats controlled the General Assembly until the 1990s.
Split into simpler sentences: This was a major reason why Virginia voted Republican in all but one presidential election from 1948 to 2004--the one exception being 1964. Meanwhile, conservative Democrats controlled the General Assembly until the 1990s.