MEXICO CITY (Reuters) - Former Mexican first lady Margarita Zavala, a leading contender to become president herself, said on Friday she was breaking with her party, deepening divisions in the opposition ahead of the 2018 presidential election.

FILE PHOTO - Margarita Zavala, lawyer and member of National Action Party (PAN), speaks during Forbes Forum 2017 in Mexico City, Mexico, September 18, 2017. REUTERS/Edgard Garrido

Zavala, wife of former President Felipe Calderon, announced her decision to leave the center-right National Action Party (PAN) in a recorded video broadcast in which she attacked the party leadership for blocking her aspirations.

Since launching her bid for the presidency in June 2015, Zavala has spent months as one of the top contenders in opinion polls, generally just behind former Mexico City Mayor Andres Manuel Lopez Obrador, a veteran leftist with nationalist leanings.

The election will be held in July 2018, and President Enrique Pena Nieto’s Institutional Revolutionary Party (PRI) faces an uphill struggle to stay in power. Polls show a tight race, though most candidates have yet to be chosen.

Zavala, 50, said she quit the PAN because the party base had been subordinated to the interests of its leadership, which has forged a cross-party alliance for the election with the center-left opposition Party of the Democratic Revolution (PRD).

“Anti-democratic conditions took hold in the PAN just like we had criticized in the PRI and other parties,” she said.

Zavala told Mexican radio that the “broad front” formed by the PAN, the PRD and the center-left Citizens Movement party meant there would be no internal PAN selection process for the presidential ticket, denying her a chance to be candidate.

The PAN has the most seats of any opposition group in Congress. Party chairman Ricardo Anaya responded with his own video message, noting the selection process was still open.

Saying he respected Zavala’s decision, Anaya nevertheless argued her departure would only benefit the PRI.

A survey in August by polling firm Mitofsky showed the cross-party alliance coming first in a presidential vote with nearly 22 percent support, a few points ahead of the PRI or Lopez Obrador’s National Regeneration Movement (MORENA).

However, over 37 percent of voters were still undecided.

Running as an independent, Zavala enters an increasingly crowded field of hopefuls that is due to include a maverick northern governor who broke with the PRI in 2015.

Fernando Belaunzaran, a leading PRD advocate of the cross-party alliance, said he regretted Zavala’s decision and that it would have been better to keep her inside the “broad front.”

If polls later show Zavala has little chance of winning, the front must be ready to welcome her back, he added. “We’ve got to the keep the door open,” he said.