Complex sentence: The Republican Party, also commonly called the GOP (for "Grand Old Party"), is one of the world's oldest political parties still in existence, the second oldest existing political party in the United States after its primary rival, the Democratic Party.
Split into simpler sentences: The Republican Party, also commonly called the GOP (for "Grand Old Party"), is one of the world's oldest extant political parties. It is the second oldest existing political party in the United States after its primary rival, the Democratic Party.