Complex sentence: The parties existing prior to the war, the Republican Party and the Democratic Party have given way to a new two party system consisting of the Confederate-influenced Whigs and the ineffectual Populists.
Split into simpler sentences: The parties existing prior to the war, the Republican Party and the Democratic Party are gone, but a two-party system of a sort nonetheless exists. Political power in the country is divided between the Confederate-influenced Whigs and the ineffectual Populists.