Interest groups and political parties form what the important policy theorists Helen M. Ingram and Anne L. Schneider called, in their influential text Policy Design for Democracy, "intermediary groups." They aggregate the opinions and sentiments of large groups of people and then represent those opinions and sentiments to policymakers.
Because these intermediary groups represent more than themselves they are better able to influence the outcome of public policy. According to Schneider and Ingram, the primary motivation of politicians is to seek and achieve reelection. Intermediary groups, representing thousands or millions of voters, can call on their supporters to give or withhold their votes to specific politicians based on their willingness to adopt policies supported by the groups themselves. This is the primary method by which interest groups and political parties influence public policy.
Other methods include cash donations to political campaigns, and the filing of amicus curiae briefs in court cases.