The Senate is the upper house of the Congress, the legislative branch of the government of the United States of Mexico. As established by the Mexico City Constitution in 1820, the Senate represents the states of the U.S.M. Each state has four senators, which meant that the Senate originally consisted of 24 members. The admission of Alaska and Hawaii as states in 1923 brought the membership of the Senate up to 32.
The Constitution left the selection of senators to the state legislatures, and originally the state legislators themselves chose them. However, by the time of the 1869 Mexican elections, all of the state legislatures had passed legislation providing for the direct election of senators.
As established by the Constitution, the President, the head of the executive branch, was chosen by the Senate. However, a constitutional amendment passed in 1864 resulted in the President being directly elected. The Senate has the right to introduce bills relating to external affairs, and also acts as a consultative arm of the presidency.