Following Mexico’s national election Sunday, will “Stop the steal” be heard as a months-long rallying cry? 

If polls are to be believed, a former Mexico City mayor, Claudia Sheinbaum, will win the presidential election by a convincing, double-digit margin. Polls, though, are notoriously unreliable, and many are watching a last-minute narrowing of the gap that could, in the least, result in an opposition leader running on a three-party ticket, Xochitl Galvez, becoming a close competitor.

Unless Ms. Sheinbaum wins by a large margin, Ms. Galvez and the parties she leads are likely to contest the outcome. “It is probably much closer than what polls show,” a former Mexican foreign minister, Jorge Castaneda, tells the Sun. “If the margin is seven points or less, the opposition is not going to concede. They will say the election was manipulated and stolen, and fight the result in court.”  

They will have a point. The Morena party that was founded by President Lopez Obrador is in many ways a throwback to Mexico’s 20th century political tradition. The oxymoronically named Institutional Revolutionary Party, or PRI, ruled the country for 71 years until 2000 — a longer single-party hold on power than enjoyed by Russia’s Communist Party.  

A PRI apparatchik until he left the party, Mr. Lopez Obrador, widely known as AMLO, then charted his own leftist, Fidel Castro-inspired political agenda. In his six years as president, he tightened his party’s grip on the courts, the press, and the unions. 

With the election nearing, the term-limited AMLO has extended his hours-long press conferences to seven days a week from five. Although a seated president is barred by law from electioneering, he has been promoting his hand-picked successor, Ms. Sheinbaum. Chatting with media supporters at the presidential palace, he also slung mud at Ms. Galvez. 

The national electoral institute, created in 1990 as an independent body after a contested election, censured Mr. Lopez Obrador 50 times for such blatant election interference. Yet, if the Morena party secures a two-thirds majority in Congress, he will be able to complete a plan to do away with the institute’s independence.

With a super majority, Morena will also be able to stack the supreme court with favored jurists, and to enact other measures meant to ensure that its one-party patronage and cronyism lasts for decades.     

A former academic specializing in energy and environmental research, Ms. Sheinbaum is widely seen as charisma-challenged. Her flat affect and seeming inability to communicate with large crowds sharply contrast her with her patron, AMLO.

Her top challenger, Ms. Galvez, is a likable, successful entrepreneur with an unassuming personality and a compelling, rags-to-riches life story. “Claudia is a scientist. She never created a job. I am a businesswoman, so I understand jobs,” Ms. Galvez told the Sun in March. 

On a level playing field, Ms. Galvez would be a much more palatable candidate to voters than Ms. Sheinbaum. Yet, while this will be a free election, “it will not be a fair one,” a former Mexican ambassador to Washington, Arturo Sarukhan, told the Georgetown Americas Institute recently. 

That view is widely shared by many Mexicans, the founder of Mexico’s Council on Foreign Relations, Andres Rozental, tells the Sun. “Obviously it’s not a balanced election when one candidate has all the governing party’s support,” including the use of public funds to “buy votes,” he says. 

Mr. Lopez Obrador “did his best to destroy our democratic institutions,” Mr. Rozental adds. Yet, he adds, it is far from clear that Ms. Sheinbaum, if she wins Sunday, would remain in AMLO’s shadow. Past presidents, he notes, have failed to exert much influence over successors. 

On the campaign trail, Ms. Sheinbaum rarely strays from the president’s messaging. Like him, she emulates the ideological grandstanding of Latin America’s “red” leaders. Ms. Sheinbaum is Jewish but last week her party, Morena, joined with South Africa’s effort at the Hague to have Israel found guilty of committing “genocide” in Gaza.    

Despite his far-left leanings, Mr. Lopez Obrador is tethered to a geographical reality: His American neighbor is Mexico’s largest trading partner. If Ms. Sheinbaum wins, she will have to maneuver between Morena’s bombastic leftism and the need to maintain close ties with Washington. 

Will Ms. Sheinbaum ever be able to emerge from AMLO’s shadow? Even if so, a Morena victory could return Mexico to the days when PRI was the only political game in town — unless Ms. Galvez manages to pull off a major upset Sunday.

“This is the 21st century,” Mr. Castaneda, who was one of the architects of President Fox’s unseating of the PRI in 2000, says. Nevertheless, Mexico could well revert to the undemocratic politics of the previous century, he says, complete with “stealing elections, a one-party system, an unfree press, and control of unions and other institutions.”