Delfina Gomez of the National Regeneration Movement (Morena), next to party leader Andres Manuel Lopez Obrador, May 16, 2017. REUTERS/Carlos Jasso

On Sunday, voters will go to the polls in the state of Mexico to pick their next governor, casting ballots in what many see as a preface to the presidential election next year — and deciding whether the long-dominant party is on the outside looking in.

While there are six candidates in the running, the race has mostly narrowed to two: Alfredo del Mazo of the governing centrist Institutional Revolutionary Party and Delfina Gomez of the upstart leftist National Regeneration Movement, both former mayors of cities there.

The state is Mexico's most populous, home to 16 million people, 11.3 million of whom can cast ballots — one in eight of the country's voters.

It is also the second-biggest contributor to the national economy, behind the capital city, with a $100 billion GDP.

The state of Mexico also has considerable political value to the PRI, which has held power there without interruption since the party's founding in 1929. Del Mazo is the son and grandson of former governors and cousin of the current president, Enrique Peña Nieto, who was governor from 2005 to 2011 and is distantly related to six former governors.

Polls have consistently shown a tight race between del Mazo, 41, and Gomez, 54, a former elementary-school teacher, who represents the three-year-old Morena.

Alfredo del Mazo, Institutional Revolutionary Party (PRI) candidate for the governor of the State of Mexico, addresses the audience at his closing campaign rally in Ecatepec in the State of Mexico, May 31, 2017. REUTERS/Carlos Jasso

Morena was formed by two-time presidential candidate Andres Manuel Lopez Obrador, a polarizing figure often viewed as a populist with nationalist leanings who broke from the center-left Party of the Democratic Revolution after his 2012 loss to Peña Nieto. In the weeks leading up to the vote, he has inveighed against the PRI and its figurehead.

"We're going to beat them here in the state of Mexico, because people have had it up to the quiff with corruption," Lopez Obrador told a rally in the town of Zinacantepec in mid-May, using the Spanish word "copete," meaning quiff, to refer to Peña Nieto's trademark carefully gelled hair.

Several polls in early May put Gomez within 1 or 2 points of del Mazo; one put her ahead by 7 points.

An El Financiero survey published on May 23 gave him the backing of 34% of those who intended to vote, while Gomez garnered 29%. A Reforma poll released May 31 showed Gomez ahead of her PRI rival, 31.9% to 30.7%. A poll done by El Universal at the end of the month showed del Mazo ahead of Gomez, 33.8% to 29.3%.

Moreover, 78% told Reforma that it was time for a party other than the PRI to run the state.

Mexican President Enrique Peña Nieto. Reuters

And while the vote is still undecided, some observers are already making declarations about the fallout.

"Nobody can deny it, today everything is centered on the State of Mexico, the state biggest in resources and in electoral register and whose result could orient candidacies, alliances and campaigns for 2018," Roy Campos, president of Consulta-Mitofsky, said in a May 24 column.

"If the PRI loses in the state of Mexico, it'll be practically impossible for it to win the presidency," Aldo Munoz, a political scientist at the Mexico State Autonomous University, told Bloomberg.

"If Delfina wins, it's virtually a formality that (Lopez Obrador) will end up as president," Fernando Belaunzaran, a PRD politician and Lopez Obrador critic who sees a Morena victory as a better than a PRI one, told Reuters.

A Morena victory on Sunday would create "the image that Mr. Lopez Obrador is unbeatable" in the 2018 presidential election, Jesus Silva-Herzog, a political scientist at the Tecnologico de Monterrey university, told The Economist.

Alfredo del Mazo, Institutional Revolutionary Party (PRI) candidate for governor of the state of Mexico, campaigns in Chalco, Mexico state, Mexico, April 29, 2017. (AP Photo/Marco Ugarte)

Del Mazo's campaign — backed by a significant war chest — has largely lived up to the reputation of the long-dominant PRI. At a recent rally in San Mateo Atenco in the state's southwest, a crowd packed a sports stadium where huge television screens broadcast his image to people who couldn't see over the banners and balloons.

Among the promises he has made are to reform the state's police force and open a new university. He has also made a play for women voters, promising a "pink salary" for housewives. Some who have benefited from PRI rule back del Mazo; others nevertheless support him because of his promises.

"I don’t like him. I like his proposals though," Karla Hernandez, a housewife at the rally, told Financial Times. "He's won us over with the pink salary. I want my pay."

A woman takes photos of graffiti of Pope Francis in Ecatepec, Mexico, February 4, 2016. REUTERS/Edgard Garrido

Many voters see the PRI as a source of handouts.

At a May rally in the city of Ecatepec, a woman showed Reuters a card with the PRI logo and a pamphlet promising a cash deposit if the PRI wins the election. Mexico’s electoral watchdog considers that vote-buying.

Gomez's Morena party has promised to end private investment in the country's oil industry, to put domestic economic interests ahead of those of foreign capital, and to repudiate the Trump administration.

Gomez, who del Mazo has attacked as inexperienced, has underlined her woman-of-the-people status and rebuked del Mazo by saying Mexico is not a monarchy — a point Lopez Obrador himself has echoed.

Gomez's standing in the governor's race has almost certainly been buoyed by Lopez Obrador — who many see as ideal to represent Mexico in its dealings with the Trump administration.

Del Mazo, however, could suffer from his ties to the PRI and Peña Nieto, whose approval rating has shrunk into the teens — driven down anger over widespread corruption, rising violence, and a struggling economy.

Delfina Gomez of the National Regeneration Movement (MORENA), candidate for the governor of the State of Mexico, addresses the audience during her electoral campaign in Metepec Thomson Reuters

A May poll by El Financiero found that 52% of respondents said insecurity was the biggest issue the next Mexico state governor needed to address.

The state's 758 homicides were the most in the country through April 2017, a year poised for record homicide levels. (As Mexico's most populous state, it typically has the most homicides.)

Some of the state's urban zones have become sites of gang fights for control of drug and extortion rackets and related violence.

In Ecatepec, which has the fifth-highest number of homicides among Mexican municipalities, five police officers were ambushed and killed early on Tuesday morning.

Ties to Peña Nieto, who has presided over significant increases in deadly violence in recent years, may not help del Mazo among security-minded voters.

The PRI, which governed every state from 1929 to 1989 and held the presidency until 2000, has seen its power erode steadily, in part because of corruption and violence. In mid-2016 state elections, the party won two of the 12 states in play , but lost seven — including four states it had governed for 86 years.

Federal police search men for drugs and weapons during an antinarcotics operation in Ecatepec, Mexico, November 13, 2009. REUTERS/Daniel Aguilar

El Financiero's May survey also found the second most respondents, 16%, saw corruption as the most important issue the state's next governor needed to address. According to the national statistics office, the state is the country's most corrupt, and poverty there rose to nearly 50% in 2014.

With that legacy, del Mazo may not fare well. Numerous PRI officials have been convicted or implicated in corrupt dealings.

In April, two former PRI governors who had been on the run from corruption charges were captured abroad. Their arrests were seen by some as a pre-election effort at redemption by Peña Nieto, especially the arrest of Javier Duarte, the former governor of Veracruz who oversaw levels of corruption and violence that outraged the country.

Whether those arrests reform the party or deepen its complicity in the eyes of voters remains to be seen.

"The immense attention Duarte's apprehension is getting here in Mexico matches the scale of the crimes he's accused of committing," Carin Zissis, editor-in-chief at the Americas Society-Council of the Americas, told Business Insider in the hours after his arrest in mid-April. "It's hard to imagine people will easily forget that his government spent six years robbing Veracruz's state coffers."

Javier Duarte, former governor of Mexico's Veracruz state, in court for extradition proceedings in Guatemala City, Guatemala, April 19, 2017. Thomson Reuters

Despite the electioneering, the election may still be decided by how many voters stay home rather than go the polls.

Turnout in recent state elections in the state has not surpassed 47%, and recent polls showed one in four of respondents had yet to pick a candidate.

"Our estimate based on surveys produces a participation between 44 and 49 percent," Alejandro Moreno, a columnist at El Financiero, wrote late last month.

While voter participation has trended up in recent elections in other states, turnout in the state of Mexico may only be about 50%, according to Campos, of Consulta-Mitofsky.

Other factors could shape the vote as well.

Morena hasn't formed alliances with any other parties for the state election. That, coupled with Lopez Obrador's broadsides at parties that haven't endorsed him, may leave opposition votes spread among candidates, allowing del Mazo and the PRI to eke out a narrow victory.

Delfina Gomez takes a selfie with supporters at her closing campaign rally in Chicoloapan de Juarez, State of Mexico, May 31, 2017. REUTERS/Henry Romero

Organizations and parties in the state have also made complaints about electoral irregularities.

Civil association #NiUnFraudeMas — Not a fraud more — said it has received 284 citizen complaints between April 6 and the end of May. Some of the irregularities the group reported were attempts to buy or coerce votes or to get public-sector sources, like transportation or health workers, to support the PRI's campaign.

Despite the parties' efforts to shape the vote, however, there are many in the state for whom the political baggage is too much to overcome.

"I'm not going to vote for anyone, they’re all thieves," Jorge Hernandez, a shoe-industry worker, told Financial Times at the del Mazo rally in San Mateo Atenco. "I don't know why people continue to back all of them when we're going to stay poor. I’ve lost faith."