Turnout: 63.42%
About this election
Held alongside López Obrador's landslide presidential win on 1 July 2018, the Chamber of Deputies election produced one of the most lopsided legislatures of Mexico's democratic era. The "Juntos Haremos Historia" coalition led by Morena won an outright majority of the 500 seats, and Morena alone became, at a stroke, the largest party in the chamber — a position the PRI had monopolised for most of the twentieth century. The result gave the incoming president the legislative power to begin his "Fourth Transformation".
The 500 deputies are elected for three-year terms through a mixed-member system: 300 in single-member districts by plurality and 200 by closed-list proportional representation in five regional constituencies. Coalitions are permitted to run joint district candidates while each member party retains its own ballot line for the proportional vote, so a coalition's seat total reflects both the districts it sweeps and the lists of its component parties. An 8-point over-representation cap is meant to keep the largest party's seat share close to its vote share, though the way coalition seats are assigned has repeatedly tested that limit.
Morena allied with the Labour Party (PT) and the conservative-evangelical Social Encounter Party (PES) under the "Juntos Haremos Historia" banner. The PAN, PRD and Citizens' Movement formed the "Por México al Frente" coalition, while the PRI ran with the Green Party and New Alliance as "Todos por México". The legislative campaign was overshadowed by the presidential contest, with López Obrador's commanding poll lead pulling Morena's congressional candidates upward across the country.
The Juntos Haremos Historia coalition won 308 of 500 seats on a combined 45.4% of the proportional vote — a clear absolute majority. Within it, Morena took 191 seats, the PT 61 and the PES 56 (the PES, having fallen below the 3% threshold nationally, lost its registration despite the seats it had been allocated). The PAN-led Frente took 129 seats (28.76%) and the once-hegemonic PRI collapsed to just 63 (24.78%), its worst result ever. The new Congress convened on 1 September 2018, three months before the president.
With majorities in both chambers, Morena and its allies were able to pass López Obrador's legislative programme, from austerity measures and social-pension guarantees to the creation of the National Guard. The scale of the defeat accelerated the decay of the PRI and reshaped the opposition. The 2018 Congress became the engine of the Fourth Transformation, though the coalition fell short of the two-thirds majority needed to amend the constitution unaided — a constraint that would shape the politics of the following six years.
Compiled and reviewed by Bartłomiej Paruzel, Election Data Analyst, from official results. See our data methodology.