Interest groups form to advocate certain ideologies and policies. Unlike political parties, they generally form around specific issues. Their ability to raise funds and affect voters makes them potentially powerful entities.
The two-party system in the United States means that both political parties have to deal with a broad range of issues. People interesting in a particular issue may find that neither party places a priority on issues they care about. Interest groups form to advocate for particular laws, regulations and policies.
One of the most powerful interest groups in the United States is the National Rifle Association, which advocates for gun rights. Its members are influenced by articles and ratings it releases, and politicians risk losing a significant voting bloc if the NRA endorses a competing politician. The NRA is also able to raise a considerable amount of funds.
Environmental interest groups are also able to influence politicians, and their fundraising ability and motivated membership causes politicians to push for legislation they endorse. In some cases, interest groups might form their own political parties. Various green parties around the world have roots in environmental interest groups, and parliamentary systems common in democratic governments often contain members of green parties.