The drafters of the Constitution created a bicameral Congress primarily as a compromise between those who felt that each state, since it was sovereign, should be equally represented, and those who felt the legislature must directly represent the people, as the House of Commons did in Great Britain. This idea of having one chamber represent people equally, while the other gives equal representation to states regardless of population, was known as the Connecticut Compromise. There was also a desire to have two Houses that could act as an internal check on each other. One was intended to be a "People's House" directly elected by the people, and with short terms obliging the representatives to remain close to their constituents. The other was intended to represent the states to such extent as they retained their sovereignty except for the powers expressly delegated to the national government. The Senate was thus not designed to serve the people of the United States equally. The Constitution provides that the approval of both chambers is necessary for the passage of legislation.
First convened in 1789, the Senate of the United States was formed on the example of the ancient Roman Senate. The name is derived from the senatus, Latin for council of elders (from senex meaning old man in Latin).
James Madison made the following comment about the Senate:
In England, at this day, if elections were open to all classes of people, the property of landed proprietors would be insecure. An agrarian law would soon take place. If these observations be just, our government ought to secure the permanent interests of the country against innovation. Landholders ought to have a share in the government, to support these invaluable interests, and to balance and check the other. They ought to be so constituted as to protect the minority of the opulent against the majority. The Senate, therefore, ought to be this body; and to answer these purposes, the people ought to have permanency and stability.
— Notes of the Secret Debates of the Federal Convention of 1787
Article Five of the Constitution stipulates that no constitutional amendment may be created to deprive a state of its equal suffrage in the Senate without that state's consent. The District of Columbia and all other territories are not entitled to representation allowed to vote in either house of Congress; though they do have official non-voting delegates in the House of Representatives, they have zero representation in the Senate. The District of Columbia and Puerto Rico each additionally elect two "shadow senators", but they are officials of their respective local governments and not members of the U.S. Senate. The United States has had 50 states since 1959, thus the Senate has had 100 senators since 1959.
Graph showing historical party control of the U.S. Senate, House and Presidency
The disparity between the most and least populous states has grown since the Connecticut Compromise, which granted each state two members of the Senate and at least one member of the House of Representatives, for a total minimum of three presidential electors, regardless of population. In 1787, Virginia had roughly ten times the population of Rhode Island, whereas today California has roughly 70 times the population of Wyoming, based on the 1790 and 2000 censuses. This means some citizens are effectively two orders of magnitude better represented in the Senate than those in other states. Seats in the House of Representatives are approximately proportionate to the population of each state, reducing the disparity of representation.
Before the adoption of the Seventeenth Amendment in 1913, senators were elected by the individual state legislatures. Problems with repeated vacant seats due to the inability of a legislature to elect senators, intrastate political struggles, and even bribery and intimidation had gradually led to a growing movement to amend the Constitution to allow for the direct election of senators.