Natural gas supply is once again at the heart of a contractual dispute between Mexico’s government and the private sector, with President Andrés Manuel López Obrador’s sights now trained on the Etileno XXI petrochemicals plant owned by Braskem Idesa in Veracruz state.

López Obrador said Monday the terms under which state oil company Petróleos Mexicanos (Pemex) supplies ethane to the plant are “leonine,” or unfair. He argued that the contract authorized under former President Enrique Peña Nieto “must be canceled, it’s my opinion, for being a leonine contract, it must be revised,” and that the government must “recover what was obtained in an illegal manner…”

The term leonine might be unfamiliar to those who do not follow López Obrador’s daily...