Mexican voters go to the polls on Sunday in an election that seems certain to deliver the country’s first female president – and may also give her party enough power in congress to change the constitution and rewire the democracy of Latin America’s second-largest economy.

Frontrunner Claudia Sheinbaum, a 61-year-old climate scientist and former mayor of Mexico City, has vowed to continue the policies of her populist predecessor, Andrés Manuel López Obrador, who founded the Morena party and forged a bond with voters who had become disenchanted with democracy.

Morena combines progressive and conservative policies in an unorthodox platform pulled together by the charisma of López Obrador and a discourse fixed on Mexico’s gaping inequality.

It has proved a winning formula – and looks set to propel Sheinbaum to victory over Xóchitl Gálvez, the leading opposition candidate.

Not just the presidency, but 20,000 other posts are up for grabs in Mexico’s biggest election ever.

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It takes place against a backdrop of violence and deepening criminal control of swaths of the country. Mexico has one of the highest homicide rates in Latin America and hundreds of organised crime groups, ranging from the small and local up to those with international presence and the kind of firepower typically reserved for armies.

These groups have diversified from drug to gun and migrant trafficking, and have penetrated local businesses and supply chains, from tortilla stands to avocado farms.

This year’s elections have been the most violent in Mexican history, with more than 30 candidates murdered and hundreds more dropping out as criminal groups vie to install friendly leaders.

On Wednesday, the final day of the campaign, a hired killer filmed himself shooting the opposition mayoral candidate José Alfredo Cabrera in the town of Coyuca de Benítez, Guerrero, before being shot dead by bodyguards.

“There is much violence – perhaps not in Mexico City, but in the rest of the country,” said Vanessa Romero, a political analyst.

Tens of thousands of supporters of Morena packed the Zócalo, Mexico City’s main square, on Wednesday for the final event of Sheinbaum’s campaign.

But the mood is less excited than in 2018, when López Obrador broke the grip that Mexico’s traditional parties had held on the presidency, winning by a landslide and promising to transform a country racked by inequality, corruption and violence.

Outside a barbershop in Benito Juárez, a wealthy neighbourhood in Mexico City, Salim said he voted for Morena in 2018 but wouldn’t this time.

“What changed? Nothing,” said Salim. “They’re the same as the others. Only the speeches are different.”

Rocío, who was visiting from the Yucatán city of Mérida, which is governed by the conservative Pan party, took a more understanding view.

“Mexico’s always had corruption,” she said. “But now it has [big infrastructure projects] like the Mayan Train – that’s a difference.”

Yet Morena’s appeal is better understood from places such as Maconí, a small community in the state of Querétaro that gets water just a few hours a week.

“We’re with Morena, and Morena is with us,” said Don José, a 66-year old man who was a migrant worker in the US for many years. “This country is changing, and the scoundrels that ruled for decades are on the way out. We would die of thirst before they paid attention to us.”

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Over the last three months, Sheinbaum has crisscrossed the country in an effort to rally such voters – not just to assure Morena retains the presidency, but to win it a level of political power not seen since Mexico became a democracy in 2000.

Morena and its allies already control a simple majority in both houses of congress and two-thirds of the governorships in Mexico’s 32 states.

The election could, in theory, gain it three more governorships on Sunday – though many of the races are too close to call, and the opposition are hopeful of wresting Mexico City from Morena’s control.

But if Scheinbaum’s party emerges with a two-thirds supermajority in congress, it would be able to amend the constitution at will.

Polling suggests it is unlikely. But Morena already has a package of reforms in mind that includes allowing supreme court justices to be elected by popular vote, which could give it control over the country’s top court.

This has led some to sound the alarm.

“Why do we have to mobilise? Because Claudia is a danger to democracy,” said Gálvez, the opposition candidate.

“I think the risk is being exaggerated for electoral reasons,” said Romero, who points out that Morena is proposing a democracy with more popular participation. “It’s not about democracy versus authoritarianism. It’s about one vision of democracy versus another.”

Others are more concerned.

“Mexico is moving to what some in Morena call ‘hegemonic democracy’, which by their definition isn’t exactly a representative liberal democracy, but nor is it authoritarian,” said Humberto Beck, a historian. “It’s something else beyond that dichotomy, which is lent legitimacy by popular support.

“I think this is an announcement of democratic regression,” added Beck.

North of the border, the Biden administration has been noticeably quiet about López Obrador’s attacks on Mexico’s institutions throughout his term, apparently prioritising cooperation on fentanyl trafficking and migration as it approaches its own election later this year.