Complex sentence: The last time it supported a Democrat for president was in 1976, when Jimmy Carter carried the district, Even that year was the only time it supported the official Democratic candidate since 1956 (it voted for George Wallace in 1968).
Split into simpler sentences: The last time it supported a Democrat for president was in 1976, when Jimmy Carter carried the district. Before then, Mississippi had not supported the official Democratic candidate since 1956 (it voted for George Wallace in 1968).