|The 1917 Constitution provides for a federal republic with powers separated into independent executive, legislative, and judicial branches. Historically, the executive is the dominant branch, with power vested in the president, who promulgates and executes the laws of the Congress. Bicameral Congress; 128-member Senate, elected for a six-year term; 64 seats are elected on a first-past-the-post basis, 32 using the first minority principle and 32 by proportional representation; 500-member Chamber of Deputies, elected for a three-year term; 300 seats are elected on a first-past-the-post basis and 200 by proportional representation. Congress has played an increasingly important role since 1997 when opposition parties first formed a majority in the legislature. The president also legislates by executive decree in certain economic and financial fields, using powers delegated from Congress. The president is elected by universal adult suffrage for a six-year term and may not hold office a second time. There is no vice president; in the event of the removal or death of the president, a provisional president is elected by Congress. Politics of Mexico takes place in a framework of a federal presidential representative democratic republic, whereby the President of Mexico is both head of state and head of government, and of a pluriform multi-party system. Executive power is exercised by the government. Legislative power is vested in both the government and the two chambers of the Congress of the Union. The Judiciary is independent of the executive and the legislature.
President of Mexico is both the head of state and head of government, as well as the commander-in-chief of the military. The president is elected directly from eligible votes and serves for six years, called a sexenio. The president is elected by direct popular vote every six years without possibility of reelection. Upon election, presidents acquire expansive authority because they also control the selection of officials in their party for political offices. Within the executive branch itself, the President has broad constitutional powers to manage national affairs and the workings of the federal government, and may issue executive orders to effect internal policies. He can declare war on other countries, but to do so, he needs the authorization of Congress. The executive branch also consists of various departments including departments for agriculture, education, and tourism.
If the presidential office falls vacant during the first two years of a sexenio, the congress designates an interim president, who, in turn, must call a special presidential election to complete the term. If the vacancy occurs during the latter four years of a sexenio, the congress designates a provisional president for the remainder of the term.
Article 50 of the Constitution grants all legislative powers of the federal government to the Congress of the Union (Congreso de la Unión). The Congress has two chambers. The Chamber of Deputies (Cámara de Diputados) has 500 members, elected for a three year term, 300 of whom are elected in single-seat constituencies by plurality, with the remaining 200 members elected by proportional representation in 5 multi-state, 40-seat constituencies. The 200 PR-seats are distributed generally without taking account the 300 plurality-seats (Parallel voting), but since 1996 a party cannot get more seats overall than 8 % above its result for the PR-seats (a party must win 42% of the votes for the PR-seats to achieve an overall majority). There are two exceptions on this rule: first, a party can only lose PR-seats due to this rule (and no plurality-seats); second, a party can never get more than 300 seats overall (even if it has more than 52% of the votes for the PR-seats).
The Chamber of Senators (Cámara de Senadores) has 128 members, elected for a six-year term, 96 of them in three-seat constituencies (corresponding to the nation's 31 states and one Federal District) and 32 by proportional representation on a nationwide basis. In the state constituencies, two seats are awarded to the plurality winner and one to the first runner-up.
The powers of the congress include the right to pass laws, impose taxes, declare war, approve the national budget, approve or reject treaties and conventions made with foreign countries, and ratify diplomatic appointments. The Senate addresses all matters concerning foreign policy, approves international agreements, and confirms presidential appointments. The Chamber of Deputies, much like the United States House of Representatives, addresses all matters pertaining to the government's budget and public expenditures.
Article 94 of the Constitution provides that the judicial power of the United , each with five justices. These are the Penal Affairs Chamber, Administrative Affairs Chamber, Civil Affairs Chamber, and Labor Affairs Chamber. A fifth chamber, the Auxiliary Chamber, is responsible for the overload of the four regular chambers. Court rulings of both the whole, or plenary, court and the separate chambers are decided on the basis of majority opinion. Rulings by the separate chambers may be overturned by the full court. In practice, the justices, along with the entire federal judiciary, traditionally submit their resignations at the beginning of each sexenio.
The most powerful juridical instrument is the writ of amparo, which can be invoked against acts by any government official, including the president. Unlike the United States system, where courts may rule on basic constitutional matters, the Mexican Supreme Court of Justice is prohibited by the constitution from applying its rulings beyond any individual case. Within this restricted sphere, the Supreme Court of Justice generally displays greater independence in relation to the president than does the legislature, often deciding against the executive in amparo cases.