MEXICO CITY — He won the presidency in a landslide. His party dominates Congress. Now, Mexican President Andrés Manuel López Obrador is in a battle over the country’s judiciary, as opponents and legal analysts accuse him of making an unconstitutional power grab.

Lawmakers from López Obrador’s party have triggered outrage by voting to add two years to the four-year term of the Supreme Court chief justice, Arturo Zaldívar. Zaldívar is generally regarded as sympathetic to the president.

As in the United States, where some Democrats want to expand the U.S. Supreme Court, there are fears that the judiciary is becoming increasingly politicized. But the Mexican measure carries especially grave implications, analysts say, because it appears to violate a constitutional limit on the chief justice’s term.

“This is the first time since our Constitution was established in 1917 that a majority of our Congress has voted in favor of a measure they know is unconstitutional,” former Supreme Court justice Diego Valadés told The Washington Post.

López Obrador is increasingly challenging institutions created as part of Mexico’s transition to democracy, including the national elections board and the freedom-of-information institute. Critics worry that the president, who came to power as a leftist political outsider, could use his popularity to reestablish elements of the one-party system that reigned here for seven decades.

“I’m not saying that a dictatorship has been installed in Mexico,” wrote Jesús Silva-Herzog Márquez, a political analyst and columnist for the daily Reforma. “What I’m saying is that our political leadership has given legitimacy to the logic, practices and values” of such a system.

Of particular concern, he wrote, members of López Obrador’s Morena party justified their support for the new measure by arguing that his transformational agenda was more important than constitutional restrictions. “I’ve never heard such a clear defense of this dictatorial logic in a discussion in the Mexican Congress,” he wrote.

The amendment to extend the chief justice’s term was added at the last minute to a judicial reform bill that was approved by the Senate earlier this month. The Chamber of Deputies passed the legislation Friday after a stormy all-night session.

Unlike in the United States, where Supreme Court justices are appointed for life, judges on Mexico’s highest court have term limits, of 15 years. Every four years, they select one member as chief justice, with no immediate reelection allowed. Zaldívar was supposed to end his term in 2022.

López Obrador has praised the new law, calling Zaldívar a “man of integrity” who needs extra time to oversee the implementation of judicial changes that are aimed at reducing widespread corruption and nepotism and improving the training of judges. The president says he does not think the new measure is unconstitutional.

Many believe what the president really wants is an ally on the Supreme Court through the end of his six-year term, in 2024.

The dispute has highlighted a growing split in Mexico over the very essence of its young democracy. López Obrador argues that many of the reforms over the past two decades have been hollow, creating the appearance of democracy even as wealthy business executives and political bosses continue to pull the strings. The result, he says, has been the entrenchment of a system of deep inequality.

If Zaldívar left the job, the president told reporters, his replacement would be “more of the same,” a justice similar to those tied to past governments. “The conservatives have created a scandal, a lot of noise with this issue, because they know what it’s about,” López Obrador told reporters this month. “It’s about judges continuing to be at the service of the mafias of economic and political power.”

The president has increasingly clashed with judges who have blocked his signature programs. He recently called for an investigation of a judge who suspended a law favoring the national electricity company over private firms. López Obrador charged that corrupt businesspeople were behind the ruling.

The National Association of Federal Circuit Court and District Judges responded that “our only commitment is to the Constitution” and to judicial independence.

Opposition parties have vowed to take the new judicial law to the Supreme Court. The chief justice has said he will recuse himself from that vote, and Valadés predicted the other 10 judges would declare it unconstitutional.

“I hope this is an isolated episode, which is not repeated, and not the beginning of a series of similar decisions,” he said. “Because that would signify a breakdown in the state of law.”

But even if the law is tossed out, some analysts say the damage has been done.

Ana Laura Magaloni, a law professor and columnist at Reforma, said the nation’s top courts were dealing with fundamental issues such as the growing power of the military and the extent of legal protections for private investment — all at a moment of deep polarization in Mexico.

“In a country in which we are all fighting, it’s very irresponsible to play around with the credibility of the ultimate referee,” she said.

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