He was elected to Congress as the federal deputy for his hometown in 1985 and, in 1987, he was selected to serve on the PRI's National Executive Committee. In 1988 Carlos Salinas chose him as the campaign manager for his (successful, although suspicious) presidential campaign. In the same election, Colosio was elected to the Senate, representing Sonora.
In the early years of Salinas's presidency, Colosio served as the chairman of their party's National Executive Committee. In 1992, Salinas chose him to serve in his cabinet, in the newly created position of Social Development Secretary. In November 1993, the PRI announced that Colosio was to be its candidate for the upcoming presidential election.
Salinas' declaration was motivated by persistent rumours that highly visible Camacho would replace Colosio, who was not doing well in his campaign. Camacho let speculation grow for some time, but eventually declared he wouldn't run for office, concentrating his attention on Chiapas situation instead. The day after Camacho's statement, Colosio was killed.
In the early evening of March 23 1994, at a campaign rally in Lomas Taurinas, a poor neighborhood of Tijuana, Baja California, Colosio was shot in the head from a distance of a few centimeters in front of a person recording video nearby. There was no foreign press coverage of this campaign event in spite of it happening only a few miles south of San Diego, California. Colosio collapsed, and was subsequently rushed to the city's main hospital, after plans to fly him to an American hospital across the border were cancelled. His death was announced a few hours later; amid contradicting eyewitness reports that remain to this day.
Although the shooter, Mario Aburto, was arrested at the site and never wavered from his story that he had acted alone, rumors still surround Colosio's assassination. The poor handling of the shooter by the authorities, who shaved, bathed and gave him a prison haircut before showing him to the media, started rumors about whether that man, who looked so different from the one arrested, was really the murderer. Colosio received three bullet wounds, and it was never clear if they could have been done by a single person or not. The case has been officially closed after many different prosecutors investigated it, but after the many mishandlings of the investigation and contradictory versions, the controversy continues.
Blame was initially laid at the feet of aforementioned Manuel Camacho, allegedly very upset at having been passed over as Salinas's successor. The finger of suspicion also pointed in the direction of organized crime, particularly the Tijuana drug cartel. With the passage of time, however, the most persistent rumor, seemingly confirmed by Ernesto Zedillo's (Colosio's campaign manager) selection as the PRI presidential candidate, is that Colosio was shot on the orders of Salinas himself because of a speech Colosio gave, talking about the Mexican people "thirst" for justice and a bright future. According to this rumor Salinas felt those words marked a rupture between Colosio and him.
Nevertheless, these conspiracy theories have proven faulty and often short of logic and have remained topics of urban mythology, as video evidence and Aburto's clinical history have proven them wrong. The shooter remains imprisoned at the high-security La Palma facility in Almoloya de Juárez, and the mystery surrounding the political assassination of the man chosen to be Mexico's president remains as open and intriguing as day one.
A few months later, Salinas' brother-in-law, José Francisco Ruiz Massieu, president of the PRI was also murdered in plain daylight in Mexico City, eliminating the two most visible and powerful official heads of the PRI in Mexico, Colosio and Ruiz Massieu. Eventually Ernesto Zedillo was elected president, becoming the last PRI president of Mexico ending the longest lasting one-party rule in modern times.
A year after Colosio's assassination, his wife, Laura Riojas, died of cancer. Prestigious newsmagazine Proceso reported Colosio's widow's first words upon learning of her husband's assassination: "Who did it?" Two children, now cared for by relatives, survived. Colosio's father continues determined to uncover what he strongly suspects are hidden truths behind his son's very public murder and, in 2004, he published a book about the case.
Mexican rock group El Tri also wrote a song, using offensive slang, about the assassination of Colosio, called Con la cola entre las patas (With the tail between the legs).