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.

Mexico's leftist leader Andres Manuel Lopez Obrador is greeted by supporters during a political gathering in Mexico City on September 22, 2013. /VCG Photo

Financial markets are closely watching Lopez Obrador's progress. If he does win in 2018, it could stoke tensions with the US 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.

Mexican President Enrique Pena Nieto delivers a speech in Mexico City, May 17, 2017. /VCG Photo

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.

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

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.

(Source: Reuters)