During its existence, the 12th district returned one deputy to the lower house of Congress for each three-year legislative session by means of the first-past-the-post system.

From 1979 onwards, votes cast in the district also counted towards the calculation of proportional representation ("plurinominal") deputies elected from the country's electoral regions.

It was dissolved by the National Electoral Institute (INE) in the 2022 redistricting process because the state's population no longer warranted 12 districts.

[7] 2017–2022 In its final form, the 12th district covered 11 municipalities in the south-west of the state: The district's head town (cabecera distrital), where results from individual polling stations were gathered together and tallied, was the city of Apatzingán de la Constitución.

[12] The 12th district's head town was at Los Reyes and it comprised 11 municipalities: