Mexico's ruling party risked humiliation Sunday in President Enrique Pena Nieto's home state as voting began in a gubernatorial election that could give a giant boost to leftist presidential hopeful Andres Manuel Lopez Obrador.
The bellwether election in the state of Mexico could prove catastrophic for the once-mighty Institutional Revolutionary Party, or PRI -- something even its own candidate admits.
Political analysts say the tight race has meanwhile bolstered Lopez Obrador's chances heading into presidential elections next year, which the fiery populist hopes will make him Mexico's first leftist president in recent memory.
His party, the National Regeneration Movement, or Morena -- an upstart leftist party that has never held a governorship -- was never supposed to come this close in the key central state.
"It's the most important state in the country and has served as the PRI's base and bastion," historian and political analyst Jose Antonio Crespo told AFP.
But the campaign in the state of 16 million people has been a mini-portrait of everything that has Mexicans fed up with Pena Nieto and the PRI.
The ruling party's candidate is Alfredo del Mazo, heir to the dynasty that has ruled the state for nearly 90 years. His father and grandfather were governors, and the president -- also a former governor -- is his cousin.
To many Mexicans, that smacks of the nepotism and back-room deals that critics say plague the PRI, a broadly centrist party that ruled Mexico as a one-party state from 1929 to 2000.
Pena Nieto brought the PRI back to power in 2012 with promises to do things differently: end cronyism, crack down on corruption and rein in the horrific violence fueled by the country's war on drugs.
Instead, his presidency has been marred by corruption scandals and a surging murder rate -- not to mention the lackluster performance of the economy, Latin America's second-largest after Brazil.
The PRI already lost five governorships in 2016, including in Veracruz, the country's third most populous state.
Facing off against Del Mazo, 41, is schoolteacher Delfina Gomez, 54, a Lopez Obrador protegee and former congresswoman who has played up her humble background as a construction worker's daughter.
"They think a little schoolteacher can't do it," Gomez has said. "These politicians think they should have all the money, that they're the only ones with the pedigree to govern."
She said Sunday that she wanted the election outcome to benefit the people, not "personal interests."
One voter, 57-year-old Georgina Pineda Lopez of Texcoco, summed up Gomez's appeal this way: "She has not bought people with anything... We know she will win and will win clean."
Even a narrow loss for Gomez would be a boon for Lopez Obrador -- widely known by his initials, AMLO -- who also ran for president in 2006 and 2012.
"There is only upside for Morena in this election," said Daniel Kerner of the Eurasia Group consultancy.
"A Gomez victory, or even narrow loss, would prove and solidify Lopez Obrador's competitiveness ahead of the 2018 presidential elections. In this scenario, the PRI would fall into a very deep crisis," he wrote in a note.
Del Mazo has acknowledged as much, saying "the future of the PRI and this country" are at stake.
"Within the PRI, they're already warning that if they lose this time, it could be the end," political columnist Martha Anaya told AFP.
Gubernatorial elections are also on in Coahuila, which borders Texas, and Nayarit, on the Pacific coast.
There, the right-wing National Action Party is hoping for wins to bolster its own prospects in next year's presidential vote.