MEXICO CITY, Aug 1 (Reuters) - Mexicans on Sunday began to vote on whether to investigate five of the country"s former leaders in a referendum championed by President Andres Manuel Lopez Obrador, amid criticism that the move is a political stunt and turnout could be low. Lopez Obrador has cast past administrations as deeply corrupt and made combating graft his top priority. But critics say he hopes to use the consultation to energize his base and that it is unlikely to muster enough votes to be valid. At least 40% of registered Mexican voters, or some 37 million people, would need to vote for the results to be binding. Analysts have said they doubt the turnout, which was slightly above 50% in June midterm elections, will be sufficient. Lopez Obrador has blamed the former leaders - Carlos Salinas, Ernesto Zedillo, Vicente Fox, Felipe Calderon and Enrique Peña Nieto, whose administrations spanned 1988 to 2018 - for aggravating many of Mexico"s woes, from poverty to insecurity. Some voters praised the effort to drum up citizen engagement but were skeptical that even a "yes" outcome would lead to punishment for past wrongdoings. "It"s a way of expressing my anger against the presidents who plundered the country, but I doubt they will really be prosecuted," said Jose Cortes, 40, before casting his ballot in Tlaxcala state. Despite widespread criticism among Mexicans of how each leader ran the country, polls show the referendum has generated little nationwide interest and most people who will vote are expected to back the proposal of the leftist president. "The consultation has become ideological," said Roy Campos, a pollster at Consulta Mitofsky. "The president"s supporters are the ones who want to go and vote, and vote yes." According to a recent survey by newspaper El Financiero, 77% of respondents said they would back the proposal to investigate former leaders, but only 31% of people said they would vote. The statute of limitations has expired for some charges that the ex-presidents could potentially face, and the referendum could lead to the creation of a truth commission rather than legal action, Campos said. Fox, who was president from 2000 to 2006 and is a vocal critic of Lopez Obrador, has urged Mexicans to stay home. "Let"s not indulge in this farce," he wrote on Twitter. The referendum asks voters to reject or back "a process of investigation of political decisions taken in past years by political actors" that would be aimed at "guaranteeing justice and the rights of possible victims." Lopez Obrador"s administration has not detailed what that process would entail. Lopez Obrador originally wanted the referendum to ask voters if they wanted the ex-presidents to be prosecuted, but the Supreme Court ordered a looser formulation to protect due process and the presumption of innocence. Some analysts say Lopez Obrador is focusing on his predecessors to distract from problems on his watch, including economic malaise, record murders and the coronavirus pandemic. "The intent is to ... place those presidents on the villains" side of history," Campos said. As well, Lopez Obrador is likely looking to rally his supporters ahead of another referendum in March, when he will hold a vote on whether to end his presidency halfway into a six-year term. "It will let him justify a way of mobilizing his base," said Antonio Ocaranza, director of OCA Reputacion consultancy.