MEXICO CITY (AP) — Mexico’s Congress debated on Sunday ahead of a vote on a constitutional reform promoted by President Andrés Manuel López Obrador that would undo much of the market opening in electrical power carried out by his predecessor in 2013.

The reform seeks to limit foreign-built renewable energy plants, and guarantee at least 54% of electricity would be bought from government-owned generating plants, which are dirtier.

The debate began with nearly all 500 deputies present. The ruling party and its allies need a two-thirds majority in order to pass the constitutional reform.

Some pro-government legislators chanted ”Traitors” at the opposition, which objects to the reform. Opposition lawmakers shouted: ”It won’t happen.”

Critics say the reform will hurt investors and their confidence in Mexico. The companies are likely to file for court injunctions, and the U.S. government may file a complaint under a free trade agreement that could result in compensatory tariffs on Mexican products.

Pro-government legislators have already passed a law giving the state utility more discretion in deciding whose electricity to buy, but it remains stalled by court challenges.

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