It's been variously described as the "world's most efficient election-winning machine" and the "perfect dictatorship" for its ability to collect the votes necessary to maintain power.
Until the defeat of its presidential candidate Francisco Labastida, Mexico's Institutional Revolutionary Party, or PRI, had never lost a presidential election in all its 71 years of history.
In terms of winning elections, the PRI was the world's most successful political party.
The Mexican Government formed the party in 1929, out of the ashes of the Mexican revolution, to maintain its power.
It adopted the national colours - red, white and green - as its own, and since then the party and government have been almost indistinguishable.
It's been variously described as the "world's most efficient election-winning machine" and the "perfect dictatorship" for its ability to collect the votes necessary to maintain power.
It did it using whatever means necessary, from vote buying to ballot stuffing to political assassinations.
In 1938, President Lazaro Cardenas began the tradition of "dedazo", or "the big finger" in which the sitting president handpicked their successor, guaranteeing a continuity of power and political ideology.
Reform calls
During the 1960s, growing calls for democratic reforms led to a massed student demonstration in Mexico city just before the 1968 Olympic games.
The PRI ordered the military in, which killed an estimated 300 students.
The incident left a scar on the country's consciousness.
1938: Tradition of appointing presidential successors begins
1946: Party renamed Institutional Revolutionary Party
1988: PRI loses 48% of its seats in the lower house
Mar 1994: Presidential candidate Luis Donaldo Colosio assassinated
Jul 1997: PRI loses control of lower house
When they were restored, he was behind.
Fraud was never proven, but many Mexicans remain convinced that the PRI stole the election.
Its next crisis came in the last elections in 1994, when its candidate Luis Donaldo Colosio was assassinated at a political rally in the northern city of Tijuana.
Democratisation
Although police caught the gunman, his motives have never been discovered. Theories range from a drug killing to an internal PRI faction that didn't want the reformist Colosio to lead the country.
Since then, Colosio's replacement and current president Ernesto Zedillo has introduced wide-ranging democratic reforms, and in 1997, the PRI lost its majority in congress for the first time.
The reforms, including a new and genuinely independent electoral institute, were widely seen as preparing Mexico for its first genuinely free elections in the country's modern history.