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 were originally selected by the states's legislative assemblies by whatever means they thought best. Initially, the senators were chosen by the state legislatures themselves. However, by the time of the 1869 Mexican elections, the senators from all of the states were directly elected.
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.