The number of Senators is set by the Constitution and cannot be changed without amending that document. The number of Representatives is set by statute and could be changed with the passage of a new public law. The House has had 435 Members since 1911, and the Senate has had 100 Members since the admission of Hawaii into the Union in 1959. When Congress began in 1789, there were only 65 Members in the House, and 26 in the Senate. In 1790, the membership of the House had grown to 106, or one Member for every 33,000 persons.

The size of the Senate is established by Article I, section 3 of the Constitution, which states: "The Senate of the United States shall be composed of two Senators from each State . . . " As for the House, section 2 of Amendment XIV to the Constitution states that "Representatives shall be apportioned among the states according to their numbers. . . " Various rulings of the Supreme Court over the years have interpreted this to mean Congressional districts should have as equal a population as possible. The Constitution also states that each state shall have at least one Representative, no matter how small its population.

Because of the Constitutional requirement that equity be maintained in the population size of each district, state legislatures must reapportion their state's population by redrawing congressional district lines every 10 years, after the results of the national Census are known. Based on the 1990 Census, the 435 statutory cap on membership in the House means each Representative has, on average, about 620,000 constituents. The next reapportionment is scheduled for 2001, after completion of the Census in 2000.

The state with the largest population, California, has 52 Representatives in Congress, while several states with small populations have only 1 Representative for the entire state. The states with so-called "At Large" Representatives are: Alaska, Delaware, Montana, North Dakota, South Dakota, Vermont, and Wyoming.

As the size of the constituency each Member must represent has steadily increased, there has been some discussion about increasing the overall size of the House of Representatives. However, more Members would also mean expanding existing office buildings and perhaps even necessitate renovations to the House chamber in the Capitol. Given the cost-conscious environment surrounding Congress in recent years, it is unlikely that proposals to spend additional money on the institution would be seen as politically viable at this time.