Understanding congress requires an understanding of its relationship with the executive branch. The basic rules of the legislative game laid out in the Constitution provide for three institutional players – the House, the Senate, and the president. Whereas the president is expected to address the needs of the nation, he can do little without Congress passing legislation creating and funding executive agencies and programs, particularly in domestic affairs. Conversely, the enactment of congressional legislation necessitates presidential approval, unless both chambers of Congress can muster a two-thirds majority to override a veto. Furthermore, the Senate must ratify treaties negotiated by the president and must approve the president's choices for top executive and judicial posts. Interdependency, based on shared as well as separate powers, characterizes the relationship among the three institutions.

Interdependency would not be important if the House, Senate, and president held similar policy preferences on all important issues. In fact, for very good reasons they often disagree about what issues should be given priority and what should be done about them. As indicated in Chapter 4, divided party control of Congress and the presidency is common. The U.S. electoral system minimizes the connection between congressional and presidential elections. Even when one party controls the House, Senate, and presidency, incumbents of the three institutions are not likely to have identical views. Representatives, senators, and presidents are elected on different cycles and they have diverse constituencies.