Complex sentence: Politically a Democrat and then an Independent in the 1970s, but a Republican since the 1980s, Giuliani was the United States Attorney for the Southern District of New York during the 1980s and prosecuted pivotal cases against the American Mafia and against corrupt corporate financiers.
Split into simpler sentences: Politically a Democrat, then an Independent in the 1970s, and a Republican since the 1980s, Giuliani was the United States Attorney for the Southern District of New York during the 1980s. Giuliani prosecuted pivotal cases against the American Mafia and against corrupt corporate financiers.