The legislative branch of national government consists of two houses – the Senate and the House of Representatives – each with a different role, different powers, and a different electoral procedure.
The House of Representatives is the institution of the federal government. The states are divided into congressional districts or constituencies of roughly equal size (around 520,000 people).
There are currently 435 members, who are elected every two years. All states must by law adopt the system of single-member constituencies with a simple majority vote. Vacancies arising from death, resignation, etc, are filled by by-elections.
The Chairman of the House of Representatives, the Speaker, is elected by the House and has important responsibilities, giving him considerable influence over the President. Moreover, should the President and Vice-President die before the end of their terms, it is the Speaker who becomes President.
The Senate is the conservative counterweight to the more populist House of Representatives.
Each state has two senators who, since 1913 (Seventeenth Amendment), have been chosen directly by the electorate. Senators are elected every six years, but the elections are staggered so that one-third of the Senate is elected every two years.
A vacancy caused by death or resignation is filled until the next congressional elections by the nomination of the State Governor.
There are currently 100 senators. The Senate has the special privilege of unlimited debate to safeguard the rights of minorities, but this can enable a small group of Senators to prevent the passage of a bill (filibustering).