Mexican President Andrés Manuel López Obrador sent a request to the Senate Tuesday for approval to place a national referendum on the ballot that would authorize investigation and prosecution of five former presidents of Mexico for crimes allegedly committed in office.

A national consultation to approve an investigation of former presidents would be unprecedented. In his announcement about the request, López Obrador said that more than two million signatures have been gathered in petitions to hold the referendum. This would be a large enough number to send the consultation to the congress for a vote. Mexico’s constitution says that a consultation can be approved by the congress based on a request from the president, a third of the members of one house of the legislature or at least two percent of the voting population.

With the president’s formal request to the Senate, this consultation proposal has now met two of those options. López Obrador called the formal request a step to provide more security, on top of the citizen signature-gathering efforts.

The consultation could lead to investigations of Felipe Calderón, Carlos Salinas, Ernesto Zedillo, Vicente Fox, and Enrique Peña Nieto. The documents submitted to the senate accuse the men of creating, between 1988 and 2018:

a period characterized by the concentration of unmeasured wealth, massive losses to the treasury, privatization of public assets, widespread corruption, flawed electoral processes and governmental practices that led to an uncontrolled growth of violence, public insecurity, massive violations of human rights, impunity as a norm and the breakdown of the rule of law in extensive zones of the nation’s territory.

Former president Calderón has said that the consultation would be a violation of due process. Calderón pointed out that if López Obrador had specific evidence against any of these former presidents, there would be nothing preventing the attorney general’s office from prosecuting them through a normal procedure, instead of a novel step like the national consultation.