Andres Manuel Lopez Obrador, leader of the National Regeneration Movement (MORENA), and Delfina Gomez, candidate for the governor of the State of Mexico, arrive to attend a news conference in Mexico City, Mexico June 9, 2017. REUTERS/Henry Romero

MEXICO CITY (Reuters) - The main rival to Mexico’s ruling party said on Friday he will seek the annulment of the vote in five districts in a key governor’s race last weekend in a bid to overturn the result and strengthen his hand ahead of next year’s presidential election.

Andres Manuel Lopez Obrador’s National Regeneration Movement (MORENA) came in a close second to the ruling Institutional Revolutionary Party (PRI) in Sunday’s election in State of Mexico, claiming it had been robbed of victory.

MORENA candidate Delfina Gomez finished some 170,000 votes adrift of PRI contender Alfredo del Mazo in the state of some 16 million people, according to a preliminary official tally.

Lopez Obrador told a news conference electoral authorities should annul five districts where the PRI was 240,000 votes ahead of MORENA due to what he claimed was voter fraud. If those districts were discounted, MORENA would be the victor, he said.

“We don’t accept the official result ... we maintain that Delfina Gomez won,” said Lopez Obrador, who has led early polls in the run-up to the 2018 presidential election.

Twice a runner-up for the presidency, Lopez Obrador has a long record of claiming voter fraud in Mexico, where elections are frequently marred by such allegations. He has yet to succeed in overturning a major vote.

Earlier this week Lopez Obrador said he would demand a vote-by-vote recount in the state but authorities have not granted that. His critics say his party is no cleaner than others.

The State of Mexico, the country’s most populous region, is home to one in eight Mexican voters and it has long been a source of strength and financing for the PRI.

The election was tarnished by widespread accusations of fraud, ranging from intimidation of voters to vote buying. Prosecutors are probing the situation.