To predict the political future one needs a grasp of the structure and processes of politics and government. Political science is the study of structure and process in government.
In the United States, political scientists look at a variety of data including elections, public opinion (on matters ranging from Social Security reform to foreign policy), institutional roles (how the U.S. Congress acts, where congressional power gravitates, how and when the Supreme Court acts, or does not act, etc.).
While historians look backward, seeking to explain the past, political scientists try to illuminate the politics of the present and predict those of the future.
Some people have questioned whether "political science" is a science at all, since "science" generally suggests a formal discipline and strict training in a systematic method, whereas much of politics is improvisation: undisciplined, informal and without system.