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Congress of the Union

Bicameral legislature of Mexico

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  • 5 chapters

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Through Bicameralism, Proportional representation and Lower house

Structure

The Congress is a bicameral body, consisting of two chambers: the Senate of the Republic and the Chamber of Deputies. Its structure and responsibilities are defined in the Third Title, Second Chapter, Articles 50 to 79 of the 1917 Constitution. The upper chamber is the Senate, Cámara de Senadores or Senado. It comprises 128 seats: 96 members are elected by plurality vote, with three members being elected in each state (two seats are awarded to the winning party or coalition and one to the first runner-up); the other 32 members are elected by proportional representation in a single country-wide constituency. Senators serve six-year terms.

The lower house is the Chamber of Deputies, or Cámara de Diputados. It has 500 seats; 300 members are elected by plurality vote and the other 200 members are elected according to proportional representation (PR), through a system of regional lists (one for each of the five constituencies established for the election by law). Deputies serve three-year terms.

The 200 PR seats are distributed generally without taking into account the 300 plurality seats (parallel voting). Since 1996, however, 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 to that rule. A party can lose only PR seats by that rule (not plurality seats). Also, a party cannot get more than 300 seats overall (even if it has more than 52% of the votes for the PR seats).

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