The margin of Claudia Sheinbaum’s election as Mexico’s president was the biggest in decades, and even as the votes were still being counted on Monday, it became clear that Mexico’s leftist governing party and its allies could be in a position to reshape the country’s political landscape.
They appear on the verge of claiming large enough majorities in Congress to enact proposals to change the Constitution that have alarmed the opposition, including advancing contentious legislation that could potentially dismantle crucial checks on presidential power.
Ms. Sheinbaum, the first woman and first Jewish person to be elected president, beat her opponent on Sunday by a stunning 30 percentage points or more, early returns show. She and her Morena party had been expected to win, but their resounding victory outperformed pre-election polls.
“We’re taking the whole shebang in these elections,” Mario Delgado, the head of the Morena party, said in a speech Sunday.
The election served as a referendum on the nearly six-year term of Andrés Manuel López Obrador, the current president, reflecting that a solid majority of the electorate has endorsed his stewardship of the country.
Preliminary results show Morena taking seven of the nine governorships up for grabs — including the most prominent, Mexico City’s — and winning supermajorities in at least 22 of the 32 state legislatures.
During Mr. López Obrador’s tenure, millions of people were brought out of poverty, the minimum wage doubled and pensions became available to many more Mexicans. But he also empowered the military, prioritized fossil fuels and pushed measures that critics say could weaken Mexico’s democratic institutions.
Still, concerns over such moves did little to sway most voters from supporting Ms. Sheinbaum, who is Mr. López Obrador’s protégée, and their party.
“Voters gave Claudia a mandate that only a very few dared to predict,” said John Feeley, deputy chief of mission at the United States Embassy in Mexico from 2009 to 2012. “Claudia cleaned house.”
For some critics, though, the ascent of a Morena party in much tighter control of both houses of Congress has already raised alarms.
“I had ruled out the possibility that Morena would have a blank check to do whatever they wanted. But that’s what we’re seeing now,” said Roberta Lajous, a Mexican diplomat who served as the country’s ambassador to four countries. “The democratic system has been used to limit democracy.”
The systemic changes Mr. López Obrador has proposed would, among other things, reduce the number of legislators in Congress; eliminate many independent regulators, transferring their functions to federal agencies; and make Supreme Court justices subject to election by popular vote. He is also seeking to make electoral officials chosen by popular vote, a measure that critics warn would weaken their independence.
The opposition in Congress had thwarted those ambitions — until now.
“There seems to be a consensus of a large part of the population to say, ‘Go ahead with your project,’” said Sergio López Ayllón, a law professor at the National Autonomous University of Mexico who has advised institutions such as the Mexican Senate and the Supreme Court.
But one piece of the puzzle is still missing for Morena and two smaller parties in its electoral alliance to exercise complete control over legislation. Early returns show the alliance will capture a two-thirds supermajority in the Chamber of Deputies, allowing it to push through constitutional amendments, but could fall short of that mark in the Senate by a few seats. If that happens, it might still be able to piece together a supermajority by making deals with other lawmakers, according to legal experts.
“They will have to negotiate something,” Mr. López Ayllón said. And if they win seats needed in the Senate, he added, “that opens a route that will lead to very fast constitutional changes.”
One unknown is how committed Ms. Sheinbaum, who will take office on Oct. 1, really is to pushing through the changes Mr. López Obrador introduced in February, and which she quickly adopted as her own. Though she has defended the proposals publicly, analysts have also said she had no choice but to wholly back Mr. López Obrador on the campaign trail.
Electoral officials are expected to announce the final results of the election sometime this week.
The Mexican peso fell more than 3 percent on Monday, a rare decline for a currency that has recently remained strong against the dollar. The jitters in financial markets reflect broader unease over a potential erosion of checks and balances, which could expand the government’s role in the economy, according to financial strategists.
A major concern for businesses is the potential “election of judges up through the Supreme Court and the virtual gutting of the independence of regulators,” said Roberta Jacobson, a former U.S. ambassador to Mexico.
The election of judges could subject them to political pressures, critics say, making them beholden to political leaders and less willing to rule against their political patrons. The move, Ms. Jacobson said, could affect companies’ ability to get a fair hearing on disputes, with either regulators or the judiciary.
“What you are doing is keeping those bodies, but taking away any fig leaf of independence by putting their functions into the executive branch,” she said of regulators. “This would take any pretense of independence away and vest that power into the presidency.”
Ms. Sheinbaum has signaled openness to working with the private sector, saying in a speech to supporters on Monday morning, “We will respect business freedom and honestly promote and facilitate domestic and foreign private investment.”
It’s possible she makes the measures a priority “given the importance of this to López Obrador’s project and legacy,” Ms. Jacobson said. But, she added: “The other possibility is that she lets it languish without ever pushing for a vote.”
Yet another scenario is raising anxiety among the government’s detractors. The new Congress will briefly overlap in September with the last month of Mr. López Obrador’s term, giving him a chance to push through structural changes if the governing party secures a supermajority in the Senate.
