MEXICO CITY (Reuters) - The leaders of two of Mexico’s main opposition parties on Saturday floated the possibility of joining forces in the 2018 presidential elections to defeat the ruling Institutional Revolutionary Party (PRI) of President Enrique Pena Nieto.

Mexico's President Enrique Pena Nieto delivers a speech during an event to recognize the contributions made by members of the Mexican foreign service, in Mexico City, Mexico, April 28, 2017. REUTERS/Edgard Garrido

The proposal by the heads of the center-right National Action Party (PAN) and center-left Party of the Democratic Revolution (PRD) could make it harder for the embattled centrist PRI to retain control of the presidency next year.

Pena Nieto, whose approval ratings have sagged to multi-year lows in recent months due to discontent over gang violence and corruption, is barred from a second term by the constitution.

A tie-up between the PAN and PRD, which together won well over 40 percent of the congressional vote in the 2012 election, could also put pressure on the current front-runner and twice runner-up Andres Manuel Lopez Obrador, who heads a new party.

“We’ve discussed the benefits of forging a broad opposition front for the 2018 elections... which would allow us to oust the PRI on the one hand, but also, more importantly, give Mexico a coalition government,” PAN chairman Ricardo Anaya told a news conference with his PRD counterpart Alejandra Barrales.

The PRD is particularly hard-pressed to compete because much of its support went to former Mexico City mayor Lopez Obrador when he broke with the party after the last election to launch a new party, the National Regeneration Movement (MORENA).

The PAN, which held the presidency from 2000 and 2012, fares better in polls, but it is struggling to gain traction for 2018.

Lopez Obrador, who has launched scathing attacks against the PAN and PRD, has shown no inclination to defer to another candidate, and bad blood between him and the parties’ leaders means they are unlikely to line up behind him.

A PAN-PRD alliance in 2018 would need to coalesce around a single candidate, and prominent figures in both parties have already signalled their desire to run.

Whether it will be possible to persuade members of one party to back the candidate of its longtime rival is uncertain, which is why some supporters of a tie-up believe an independent “citizen” candidate would be the best option.

However, few independents have much national visibility so far, and Lopez Obrador will win additional momentum if the MORENA candidate can win a June gubernatorial contest in the State of Mexico, the country’s most populous state.

Polls show MORENA could oust the PRI, which has ruled the state next to Mexico City for nearly 90 years.