| MEXICO CITY
MEXICO CITY Struggling to halt a run of electoral losses, Mexico's ruling party squares off against its main leftist rival in a major state election on Sunday that could prove a dry run for next year's presidential contest.
President Enrique Pena Nieto's Institutional Revolutionary Party (PRI) is battling to defend its biggest state bastion from the new party of veteran campaigner Andres Manuel Lopez Obrador, who has led early polls for the July 2018 presidential race.
In PRI hands since 1929, the State of Mexico home to one in eight Mexican voters, and if it falls to Lopez Obrador's leftist National Regeneration Movement (MORENA) it could provide him with a springboard to take the top job.
"It's a pivotal election, not just for MORENA, it's a pivotal election for Mexico," the two-time presidential runner-up said in a recent radio interview. "Imagine the message that will go out to the world (if MORENA wins)."
Victory for the combative Lopez Obrador in 2018 could push Mexico in a more nationalist direction at a time of heightened tensions with the United States. President Donald Trump has riled Mexicans with threats to tear up a joint trade deal and build a border wall to keep out undocumented immigrants.
Opinion polls show MORENA's gubernatorial candidate for the State of Mexico, Delfina Gomez, running neck-and neck with PRI rival Alfredo del Mazo in the region of 16 million people that Pena Nieto himself once governed.
Pena Nieto's popularity helped his successor retain the state by a landslide in 2011, but his 4 1/2 years as president have battered his reputation and hurt the party.
Failing to end corruption scandals that have long tarnished the PRI, and struggling to tame brutal gang violence, the president is no longer an electoral asset, while his home state has become a symbol of all that plagues his administration.
Polls show most voters in the State of Mexico, which is on the edge of Mexico City, want a new government but are divided about who should form it.
"People here need a change, whatever it is, but the PRI has to go," said 42-year-old school teacher Rosario Cruz at a closing rally for MORENA's Gomez, herself an ex-teacher.
The PRI also defends two other governorships on Sunday, in the states of Nayarit and Coahuila. Polls show the PRI is trailing well behind the main opposition candidate in Nayarit, and is in a tight race in Coahuila, where it could be ousted for the first time.
Under Pena Nieto, the party's once iron hold on Mexico's states has gradually weakened, and it now has five fewer governors than when he took office. Going into Sunday, the PRI and its allies controlled 16 states, or half the regional governments.
The State of Mexico is the jewel in the PRI's fading crown, and the party is not giving it up without a fight.
Opposition accusations of PRI vote-buying are rife.
"They're doing what they did in 2011, but on a bigger scale and in greater volume," said Alejandro Encinas, a leftist opposition senator who finished second in the 2011 contest for the state.
"The penetration of money is quite something where there's so much poverty. You have whole families who sell their vote."
The PRI rejects the accusations.
Lopez Obrador's party has yet to win a state election, and the PRI this week seized on tweets by the Venezuelan embassy thanking MORENA for support to accuse him of wanting to turn Mexico into struggling Venezuela.
The Venezuelan embassy later deleted the tweets, but the episode revived memories of the last two presidential races, when adversaries painted Lopez Obrador as a Mexican Hugo Chavez who threatened the economic stability of the nation.
That fear is already in the minds of some voters.
"The only person I don't want to have ruling Mexico is Andres Manuel Lopez Obrador," 58-year-old PRI supporter Maria Asuncion Garcia said at the party's closing rally in Ecatepec, State of Mexico. "Because Venezuela was a prosperous country until populism descended on it."
(Reporting by Dave Graham, additional reporting by Anahi Rama; Editing by Andrew Hay)