The 1917 constitution provides for a federal republic with powers separated into independent executive, legislative, and judicial branches. Historically, the executive has been the dominant branch, with power vested in the president, who promulgates and executes the laws of the Congress. The Congress has played an increasingly important role since 1997, when opposition parties first made major gains. The president also legislates by executive decree in certain economic and financial fields, using powers delegated from the Congress. The president is elected by universal adult suffrage for a 6-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 the Congress.
The Congress is composed of a Senate and a Chamber of Deputies. Consecutive re-election is prohibited. Senators are elected to 6-year terms, and deputies serve 3-year terms. The Senate's 128 seats are filled by a mixture of direct-election and proportional representation. In the lower chamber, 300 deputies are directly elected to represent single-member districts, and 200 are selected by a modified form of proportional representation from five electoral regions. The 200 proportional representation seats were created to help smaller parties gain access to the Chamber.
The judiciary is divided into federal and state court systems, with federal courts having jurisdiction over most civil cases and those involving major felonies. Under the constitution, trial and sentencing must be completed within 12 months of arrest for crimes that would carry at least a 2-year sentence. In practice, the judicial system often does not meet this requirement. Trial is by judge, not jury. Defendants have a right to counsel, and public defenders are available. Other rights include defense against self-incrimination, the right to confront one's accusers, and the right to a public trial. Supreme Court justices are appointed by the president and approved by the Senate.