Legislative branch: description: bicameral National Congress or Congreso de la Union consists of the Senate or Camara de Senadores (128 seats; 96 members directly elected in multi-seat constituencies by simple majority vote and 32 directly elected in a single, nationwide constituency by proportional representation vote; members serve 6-year terms) and the Chamber of Deputies or Camara de Diputados (500 seats; 300 members directly elected in single-seat constituencies by simple majority vote and 200 directly elected in a single, nationwide constituency by proportional representation vote; members serve 3-year terms)
note: for the 2018 elections, senators will be eligible for a second term and deputies up to 4 consecutive terms
elections: Senate - last held on 1 July 2012 (next to be held on 1 July 2018); Chamber of Deputies - last held on 7 June 2015 (next to be held on 1 July 2018)
election results: Senate - percent of vote by party - NA; seats by party - PRI 52, PAN 38, PRD 22, PVEM 9, PT 4, MC 2, PANAL 1;
Chamber of Deputies - percent of vote by party - NA; seats by party - PRI 203, PAN 108, PRD 56, PVEM 47, MORENA 35, MC 26, PANAL 10, PES 8, PT 6, independent 1
Definition: This entry has three subfields. The description subfield provides the legislative structure (unicameral â€“ single house; bicameral â€“ an upper and a lower house); formal name(s); number of member seats; types of constituencies or voting districts (single seat, multi-seat, nationwide); electoral voting system(s); and member term of office. The elections subfield includes the dates of the last election and next election. The election results subfield lists percent of vote by party/coalition and number of seats by party/coalition in the last election (in bicameral legislatures, upper house results are listed first). In general, parties with less than four seats and less than 4 percent of the vote are aggregated and listed as "other," and non-party-affiliated seats are listed as "independent." Also, the entries for some countries include two sets of percent of vote by party and seats by party; the former reflects results following a formal election announcement, and the latter â€“ following a mid-term or byelection â€“ reflects changes in a legislatureâ€™s political party composition.
Of the approximately 240 countries with legislative bodies, approximately two-thirds are unicameral, and the remainder, bicameral. The selection of legislative members is typically governed by a country's constitution and/or its electoral laws. In general, members are either directly elected by a country's eligible voters using a defined electoral system; indirectly elected or selected by its province, state, or department legislatures; or appointed by the country's executive body. Legislative members in many countries are selected both directly and indirectly, and the electoral laws of some countries reserve seats for women and various ethnic and minority groups.
Worldwide, the two predominant direct voting systems are plurality/majority and proportional representation. The most common of the several plurality/majority systems is simple majority vote, or first-past-the-post, in which the candidate receiving the most votes is elected. Countries' legislatures such as Bangladesh's Parliament, Malaysia's House of Representatives, and the United Kingdom's House of Commons use this system. Another common plurality/majority system â€“ absolute majority or two-round â€“ requires that candidates win at least 50 percent of the votes to be elected. If none of the candidates meets that vote threshold in the initial election, a second poll or "runoff" is held soon after for the two top vote getters, and the candidate receiving a simple vote majority is declared the winner. Examples of the two-round system are Haiti's Chamber of Deputies, Mali's National Assembly, and Uzbekistan's Legislative Chamber. Other plurality/majority voting systems, referred to as preferential voting and generally used in multi-seat constituencies, are block vote and single non-transferable vote, in which voters cast their ballots by ranking their candidate preferences from highest to lowest.
Proportional representation electoral systems â€“ in contrast to plurality/majority systems â€“ generally award legislative seats to political parties in approximate proportion to the number of votes each receives. For example, in a 100-member legislature, if Party A receives 50 percent of the total vote, Party B, 30 percent, and Party C, 20 percent, then Party A would be awarded 50 seats, Party B 30 seats, and Party C 20 seats. There are various forms of proportional representation and the degree of reaching proportionality varies. Some forms of proportional representation are focused solely on achieving the proportional representation of different political parties and voters cast ballots only for political parties, whereas in other forms, voters cast ballots for individual candidates within a political party.
Many countries—both unicameral and bicameral—use a mix of electoral methods, in which a portion of legislative seats are awarded using one system, such as plurality/majority, while the remaining seats are awarded by another system, such as proportional representation. Many countries with bicameral legislatures use different voting systems for the two chambers.
Source: CIA World Factbook - This page was last updated on January 20, 2018