The Institutional Revolution Party or PRI, dominated political control over Mexico from the late 1920's until the 2000 elections. There are many reasons why the PRI was able to maintain power for so long including corruption, manipulation and taking advantage of patronage to manipulate support. I feel in order to understand the PRI's control over Mexican politics, you have to understand how they came to power.
The PRI came to power during the end of the Mexican Revolution. Various leaders decided to unite their armies to form a Mexican army and share joint control and power over Mexican affairs. This alliance was originally formed as the National Revolutionary Party but eventually became to PRI. They took absolute control over the rule of Mexico and did not have that control diminished until the 2000 Presidential elections, where Vicente Fox Quesada from the PAN party won power. This election was viewed by outside observers, to be the most legitimate election in Mexico's history.
There are many reasons why the PRI has dominated Mexican politics but I feel the most important reason is corruption. The PRI has a long history of manipulating elections or results to their parties favor. This corruption became so blatant and obvious that it was often accepted as normal. The PRI used their influence to rig elections or to control who could vote or what votes where counted.
The PRI also used a patronage system to insure power and control. Those who were loyal to the party received jobs or positions within the government. Some that did not receive direct jobs within the government where awarded contracts and jobs to do work for the government. They used these types of rewards to keep party supporters happy and loyal.
The PRI also had a system in effect where the President would nominate their successor, after discussion from their cabinet and important officials. This type of nomination would eliminate much of the infighting in the party. The new leader would basically be groomed to take control from those in power. Once the new candidate was selected they already had the strong political base needed for control. The nominee would also start to exert some control over political decisions before they are even elected.
The PRI also has had a long history of taking advantage of the poor and underprivileged members of Mexican society. The PRI would often use little gifts or bribes to manipulate the poor and pheasants to vote in their favor. The pheasants often had no idea what they were voting for and often could not read or right. The PRI would essentially do everything the person needed to place to vote and let the pheasant do that part.
The PRI also maintained control of the working class in Mexico, which was another key to their longevity. The PRI would take steps to insure better wages and conditions for the workers. These small steps by the PRI were able to maintain the PRI in favorable conditions with the working class and therefore they were less concerned about corruption within the government.
Combining control over the pheasants and the working class along with control of the Bureaucracy allowed the PRI to have what was known as an "iron triangle". With control over these three important sectors of Mexican society, the PRI was able to control the base that would keep the in power for seventy years.
The PRI maintained dominate control over Mexico politics for a variety of reasons that I tried to address but I feel the most important reason would be the corruption that plagued their system. The PRI was able to manipulate and fix elections in their favor and therefore maintain a much stronger hold of power. While I do feel that corruption is the main reason for the PRI's dominance, the other factors discussed still had an important role. The working class and pheasants would not have accepted the corruption had they not been getting there needs addressed for example. I believe that the PRI was able to maintain a strangle hold on Mexican politics for so long because they combined all of these little efforts to achieve the ultimate goal, which was control.

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