Evaluate the roles of public opinion, interest groups, and political parties in the political system.

In a large, democratic state it is difficult for individuals to directly influence the design of public policy. Interest groups and political parties - collectively referred to as “intermediate groups” and, in some cases, “civil society organizations” – occupy a middle space between the public (represented by the state) and the private (represented by the individual) spheres. They aggregate public opinion and use their influence as representatives of significant segments of the population to press the public sphere to enact policies and laws beneficial to the private.
In the early history of the United States, the role of interest groups and political parties was viewed with suspicion and skepticism. The ideals of Madisonian democracy considers these intermediate groups to be detrimental to the proper functioning of a democratic state out of concern that they will seek gains for their own sake. However, in contemporary times, the concept of political pluralism considers interest groups and political parties necessary to democratic governance by providing a conduit for communication between the state and the people.
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Policy Design for Democracy

Elected officials are always concerned with public opinion, which is why political scientists have developed so many tools for measuring it. Politicians will often test the popularity (or unpopularity) of potential legislation before undertaking it. This is obviously because, in a democratic society, politicians can be voted out of office if they are unpopular. But politicians are not totally at the mercy of public opinion—they can help shape it through spending on advertisements, appearances on news broadcasts, and public speaking engagements.
Interest groups are very important in American politics. They can take several forms, but the most prominent in the United States are interest groups that represent the interests of a single group. Their role is to ensure through raising money, hiring lobbyists, and conducting publicity campaigns, that the issues that concern their members are addressed in national and state politics. The National Rifle Association (NRA) is a very powerful and controversial interest group. They use their influence to oppose restrictions on guns and to promote the interests of gun owners.
The role of political parties is to get their members elected. The two major parties in the United States stand on two different ideological platforms. These sets of beliefs are generally shared by their members and by the people that the parties support in elections. Parties nominate candidates through primary elections and back these candidates financially. Ideally, a party's candidate is one who has been "vetted" by the party and who party supporters can feel comfortable voting for. Because of the expense of American politics, as well as the primary system and the "winner-take-all" system of elections, it is very difficult for people outside the two major parties to get elected.