"AMLO," as he is widely known, has a double-digit lead at the start of the race to succeed President Enrique Pena Nieto, whose popularity has been flayed by corruption scandals, a seemingly hopeless war on drug cartels and record-shattering crime that has left a trail of bodies in its wake.

But Lopez Obrador, a sometimes fiery leftist who is making his third presidential bid, has been here before: in 2006, he was the front-runner for most of the race, then narrowly lost to Felipe Calderon of the conservative National Action Party (PAN).

Two rivals are meanwhile fighting a no-holds-barred battle for second place.

Ricardo Anaya of the PAN is a youthful ex-lawmaker whose bid to campaign as a fresh face has been blotched by accusations of corruption and strongarming his way to his party's nomination.

Jose Antonio Meade is a respected former finance minister standing for the ruling Institutional Revolutionary Party (PRI) -- a long-dominant force in Mexican politics whose popularity is now so low it tapped a non-party member as its candidate.

Running a distant fourth is independent Margarita Zavala, ex-president Calderon's wife, who quit the PAN in a bitter dispute with Anaya and is now peeling away a potentially crucial part of his vote.