The primary difference between a the purpose that each serves. political parties exist to gain power over governmental policy by winning elections for political office. they do this by supporting candidates for offices and them win elections through advertising and fundraising. they have official opinions on a wide variety of issues, but these are subject to change. an interest group's goal is to promote a position on a specific issue such as gun control or agriculture. interest groups do not necessarily have their members run for office, and they vote in a nonpartisan way, supporting candidates who promote their point of view.