Mexico's federal legislature is called the Congresso de la Union. It consists of two chambers, the Camara de Diputados (Chamber of Deputies), and the Camara de Senadores (Senate).
We have created a table of votes and seats won by parties in the July 2003 elections.
The Chamber of Deputies
The Chamber of Deputies has 500 members, who are elected for three year terms . Representatives cannot serve for two consecutive terms, although they can serve again after an intermediate term.
Of its 500 members, 300 are elected by plurality in single member districts, and 200 are elected by proportional representation using a party list system.
No political party is allowed to have more than 300 of the 500 seats in the Chamber of Deputies. (Therefore no party can gain the two-thirds majority needed to change the constitution by itself.)
Also, no political party is allowed to hold a share of seats in the Chamber of Deputies which is more than 8% larger than their share of the total national vote, unless they earn all of their seats in the single member district elections.
For example, if a party wins 40% of the national vote, it cannot have more than 48% of the 500 seats (that is, 240), unless it wins all of those 240 seats in the single member district elections (which is unlikely to happen).
This places a limit on the disproportionality of the final outcome, and makes the Mexican system in theory slightly more proportional than other parallel systems, which elect some seats by single member plurality and others by propurtional representation, without adjusting the number of proportional representation seats awarded to compensate for dispropotionalities in the single member plurality elections. (For example, Japan and Russia.)
However, it is perhaps less proportional than systems than other "mixed member proportional" systems, which aim to use the proportional representation component to compensate for all of the disproportionality in the single member component, rather than coming within an 8% range of doing so. (For example, Germany and New Zealand.)
The proportional representation component of the election is carried out by means of regional lists in five multi-member districts, which each elect 40 members.
In order to participate in the Chamber of Deputies election, a party must demonstrate that it has registered candidates for at least 200 of the 300 single member seats.
Party lists are ranked. For example, if a party gains 10 seats in a multi-member district, the first 10 candidates on the party's list are given seats. Lists in Mexico are closed , which means that voters cannot affect the order of their chosen party's list.
Parties can register up to 60 candidates as contestants for both single member and party list elections.
Parties must attain at least 2% of the regional votes cast in the multi-member districts to be eligible for the proportional representation seats.
The Senate has 128 members, who are elected for six year terms . Senators cannot serve for two consecutive terms, although they can serve again after an intermediate term.
Three Senators are elected in each one of the 32 states within Mexico, which accounts for 96 of the 128 seats.
In each of these states, parties set up a two-candidate list to be voted upon. Two of the three seats is given to the party with the most votes, and the remaining one seat is given to the party with the second most votes.
The remaining 32 seats are
elected by proportional representation in one national multi-member district
Researched by James Green-Armytage, as associate of the Center for Voting and Democracy, in August 2003.