A congressional caucus is a group of members of the United States Congress that meets to pursue common legislative objectives. Formally, caucuses are formed as congressional member organizations (CMOs) through the United States House of Representatives and the United States Senate and governed under the rules of these chambers. In addition to the term caucus, they are sometimes called conferences (especially Republican ones), coalitions, study groups, task forces, or working groups. Many other countries use the term "parliamentary group"; for example, the UK Parliament has many all-party parliamentary groups. 
The largest caucuses are the party caucuses and conferences in the United States Congress, which are the partisan caucuses comprising all members of one house from one party (either the Democrats or the Republicans) in addition to any Independent members who may caucus with either party. These are the House Democratic Caucus, House Republican Conference, Senate Democratic Caucus, and Senate Republican Conference.
The caucuses meet regularly in closed sessions to set legislative agendas, select committee members and chairs, and hold elections to choose various floor leaders. They also oversee the four Hill committees, political party committees that work to elect members of their own party to Congress.
Some caucuses are organized political factions with a common ideological orientation:
Most ideological caucuses are confined to the House of Representatives.
|Congress||CPC||NDC||BDC||Democrats (in total)||Republicans (in total)||TG||MSP||RSC||LC||HFC||Reference|
^ Total counts may vary, as members are not limited to membership in a single caucus. Also the provided numbers are those of known members of their respective caucuses - this does not necessarily reflect the true numbers (which can easily be higher).
The most common caucuses consist of members united as an interest group. These are often bipartisan (comprising both Democrats and Republicans) and bicameral (comprising both Representatives and Senators). For example, the Congressional Bike Caucus works to promote cycling.
The House Committee on House Administration prescribes certain rules for Congressional Member Organizations (CMOs). Each Congress, CMOs must electronically register with the Committee on House Administration, providing the name of the caucus, a statement of purpose, the CMO officers, and the employee designated to work on issues related to the CMO.
The Committee on House Administration rules include: