Mexico is a federal republic consisting of 31 states and the Federal District. Its basic political institutions are defined in the constitution of 1917.
The president, elected for a six-year term (by universal adult suffrage beginning at age 18) and forever ineligible for reelection, appoints the attorney-general and a cabinet, which may vary in number. Although the constitution established separation of powers, in practice the Mexican chief executive dominates the legislative and judicial branches. Since the president is head of state, head of government, and commander-in-chief of the armed forces, and since his party, the PRI, has enjoyed a clear majority in Congress since it was created, it has been said that the only limit placed on the power of a president of Mexico is that of time—six years in office. With the approval of Congress, the president may intervene in the states, restricting their independence; also, under congressional authorization, he has certain legislative authority, especially in the regulation and development of commerce and industry. There is no vice president. If the president dies or is removed from office, Congress is constitutionally empowered to elect a provisional president.
The bicameral Congress, also elected by direct universal suffrage, is composed of a Senate (Cámara de Senadores), expanded from 64 to 128 members in 1994 (four from each state and four from the Federal District), and a Chamber of Deputies (Cámara de Diputados) made up of 500 members; one deputy (and one alternate) represents each 250,000 people or fraction above 100,000, with a minimum of two deputies from each state, and 200 members are allocated by proportional representation from minority parties. Senators are elected for six years (half the Senate is elected every three years) and deputies for three years, and both groups are ineligible for immediate reelection. Congress meets from September through December; for the other eight months of the year there is a permanent committee consisting of 14 senators and 15 deputies. The Congress may legislate on all matters pertaining to the national government and the Federal District.
In an effort to unite various interest groups within the government party, a National Consultative Committee, composed of living ex-presidents of Mexico, was formed in 1961 by President Adolfo López Mateos (1958–64). In the final 1962 congressional session, legislation was passed to reform the 1954 electoral law. The bill was designed to give minority parties a greater chance for representation in the Chamber of Deputies by allowing them five seats if they received 2.5% of the total vote and one additional seat for each 0.5% of the vote beyond that. Under this measure, a small party could obtain up to 20 seats without winning in a single district. A new electoral reform, introduced in 1977, stipulated that the minimum number of members must be 65,000 for a party to be registered and that the party must receive 1.5% of the popular vote to have its registration confirmed. In 1986, Congress stipulated that 200 deputies would be elected by a system of proportional representation within multimember constituencies, while 300 deputies would be elected by majority vote within single-member electoral districts. The electoral formulas for proportional representation in the Senate have changed repeatedly during the past 10 years to accommodate the growing influence and power of the PAN and PRD.