The United States has a federal government, which is divided into three branches: The Legislative branch, whose highest organization is The Congress. The Executive branch, led by The President, and the Judicial Branch, ruled by The Supreme Court. All of these organizations influence in each other's decisions. Also, the senators and representatives that work in The Congress have a staff that works for them and have a little influence.
However, the constituents (or voters) of each electoral district probably have the least influence because they don't take part directly in the Congress. Anyway, the constituent role is still important, because they are the ones who elect the senators and the representatives.