Electoral regions of Mexico

The electoral regions (circunscripciones electorales) of Mexico are geographic areas composed of various states used for the election of the 200 proportional representation legislators to the Chamber of Deputies.

Mexico is split into five separate regions. Each of the five regions elects 40 deputies, who are selected according to party lists in the regions. To distinguish them from those elected in the 300 single-member constituencies, they are often referred to as "plurinominal deputies".

The electoral regions are divided to ensure a roughly equal distribution of population among the regions and may be modified by the National Electoral Institute. The current distribution has been in use since September 30, 2005.

In contrast, the 32 senators-at-large elected to the Senate by the principle of proportional representation are drawn from party lists covering the entire country; the electoral regions therefore play no role in Senate elections.

Historical evolution
The electoral region mechanism was introduced as part of the 1977 political reforms. First used for the 1979 legislative election, there were at first only three regions, which elected 100 plurinominal deputies. For the 1982 general election, the number was increased to four, and the fifth region was introduced for the 1985 mid-terms. Following further reforms in 1986, the number of plurinominal seats in the Chamber was increased to 200 for the 1988 election.