Mexico’s ruling Morena party and its allies lost their supermajority in the lower congressional house in Sunday’s midterm elections, placing a check of sorts on President Andrés Manuel López Obrador’s nationalist energy agenda.


Preliminary results showed that the president’s coalition still holds simple majorities in the senate and the chamber of deputies, which is enough to pass legislation.

Amending the constitution, however, requires approval of two-thirds majorities in each chamber, along with simple majorities in at least half of the state legislatures.

Proponents of Mexico’s market-opening 2013 energy reform had expressed concern about López Obrador, aka AMLO, seeking to amend the constitution in order to give Mexico’s state-owned energy firms a leg up...