Maria Cristina Diaz , right, is the new interim leader of the PRI following Humberto Moreira's, left, resignation [Reuters]

The head of Mexico's former ruling party has resigned amid a financial scandal that is threatening to wreck the party's chances of returning to power and to rebrand itself as fraud-free.

Humberto Moreira, who headed the Institutional Revolutionary Party (PRI), stepped down on Friday at a party meeting broadcast nationwide and intercut with live denunciations by opposition politicians.

Moreira had widely been promoted as the face of the newly reformed PRI after he quit as governor of the northern state of Coahuila last January.

Then, in July, the Coahuila legislature said the state's total debt was four times larger than the $700m that was reported by state officials just before Moreira stepped down.

The ruling National Action Party (PAN) said it suspected at least some of the public money was stolen by officials, demanding a criminal investigation into the assets of one of Moreira's former aides.

Moreira has not clearly explained the soaring debt figure, but has said repeatedly that the debt issue is being used by PAN as a smear campaign.

"I've resigned because I'm not going to allow a media war that is trying to harm our party to continue," Moreira told party members on Friday.

Criminal charges

The PRI was in power for seven decades until voters angry at economic mismanagement, cronyism and corruption chose the conservative National Action Party in the 2000 presidential race.

After a little more than a decade, Mexicans appear to widely accept the PRI's argument that it has become democratic.

Enrique Pena Nieto, the PRI's youthful presidential candidate and former Mexico State governor, leads potential competitors by double digits in recent opinion polls on the forthcoming elections.

For months, Pena Nieto and other powerful PRI members stood by Moreira as their party leader, who repeatedly said he would not step down.

Then, on Monday, Coahuila's state treasurer was arrested on suspicion of falsifying state documents that authorised the government to seek new loans - the first criminal charges in the case.

Pena Nieto and other PRI members began distancing themselves from Moreira, and on Thursday the presidential candidate told Milenio Television that the party "clearly needed to weigh the circumstances of the weakening of our party's leader".