By Anahi Rama

ECAPTEPEC, Mexico (Reuters) - Nine decades of rule by President Enrique Pena Nieto's Institutional Revolutionary Party (PRI) in Mexico's most populous state are hanging in the balance in an election that could batter its hopes of keeping power nationally in 2018.

Polls show the National Regeneration Movement (MORENA), the new party of veteran leftist Andres Manuel Lopez Obrador, could wrest control of the state of Mexico from the PRI by winning the governorship in the June 4 state election, a result that would ramp up the momentum for his bid to succeed Pena Nieto in 2018.

Headstrong and with more nationalist leanings than the centrist PRI, two-time presidential runner-up Lopez Obrador has led early opinion polls for next year's contest.

Financial markets are closely watching Lopez Obrador's progress. If he does win in 2018, it could stoke tensions with the United States after President Donald Trump's populist broadsides against Mexico during his own election campaign.

Castigating Pena Nieto for his government's failure to stamp out political corruption and rising gang violence, Lopez Obrador has sought to turn the state campaign into a referendum on PRI rule in the biggest remaining bastion of the ruling party.

"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 last week in the town of Zinacantepec, using the Spanish word "copete" (quiff) to refer to Pena Nieto's trademark carefully gelled hair.

Pena Nieto hails from the state on the edge of Mexico City, where one in eight of the country's voters live. Before becoming president, he was governor and then helped his PRI successor win election with more than 60 percent of the vote in 2011.

TIGHT POLLS

However, surveys show the PRI struggling to muster half that this time.

A survey by newspaper Reforma in late April showed PRI gubernatorial candidate Alfredo del Mazo with the backing of 28 percent of voters, one percentage point behind Delfina Gomez of Lopez Obrador's MORENA party.

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Other surveys also predict a close finish with the center-right National Action Party some way back in third place.

The PRI must also defend two other governorships in next month's elections in the states of Nayarit and Coahuila.

Shaken by the risk of losing the state that is home to 16 million people, the PRI has sought to depict Gomez as inexperienced and personally corrupt. A former teacher, Gomez was a city mayor in the region for two years.

The PRI also is throwing money at the problem. Many people at a del Mazo rally in the city of Ecatepec were drawn by the promise of handouts. One woman showed a card emblazoned with the PRI logo and a pamphlet promising a cash deposit if the party wins the election - a practice the electoral watchdog considers vote buying.

Many PRI supporters just shrug their shoulders.

"Better the devil you know," Maria de los Remedios Gonzalez, 49, said at the rally. "If you don't get close to them you get nothing. At the end of the day, it's our money, and they give us a bit back."

Some attending said they would not even vote for del Mazo.

"This election is between the PRI and MORENA. I'm for a change, personally," said mother-of-four Jovana Medina, 44. "But in general, I think they're all a bunch thieves."

GAME CHANGER

Four-and-a-half bruising years as president have hammered Pena Nieto's approval ratings and worn down the PRI, which emerged as the nation's dominant political force after the 1910-1920 Mexican Revolution.

The party's choice of del Mazo as candidate - he is both Pena Nieto's cousin, as well as the son and grandson of former PRI governors of the state - has loaded its campaign with political baggage and undercut the message of renewal.

Pena Nieto himself is barred by the constitution from seeking re-election as president in 2018.

Now 63, Lopez Obrador has likened del Mazo's candidacy to a "monarchy," and he and other opposition leaders have attacked the PRI for its cozy relationship with businesses in the state that they say helped fund its electoral campaigns.

The silver-haired former Mexico City mayor also has lashed out repeatedly at rival parties that refuse to support him, feeding PRI hopes that divisions in the opposition will enable it to scramble enough votes together to hold on to the state.

Lopez Obrador, who has a history of accusing rivals of cheating, has already warned supporters that the PRI will resort to fraud to claim victory in a state long synonymous with unscrupulous politics - but also the party's success.

An early incarnation of the PRI took hold of the region shortly before the Wall Street Crash of 1929, and for the next 60 years, the party ran every Mexican state.

But since its first state loss in 1989, the opposition has gradually whittled away its regional bases, leaving the state of Mexico as the nerve center of the PRI political machine.

That makes the state a game-changer, said Fernando Belaunzaran, a politician in the center-left Party of the Democratic Revolution and trenchant Lopez Obrador critic, who nonetheless argues a MORENA triumph is preferable to the PRI.

"If Delfina wins, it's virtually a formality that (Lopez Obrador) will end up as president," he said.

(Additional reporting by Dave Graham; Editing by Bill Trott)