Mexico's PRI and its forerunners dominated electoral politics from 1929 until it lost its majority in Congress in 1997 and lost the presidency in 2000; however, single-party dominance did not mean the absence of opposition parties. Opposition forces were allowed to register as parties and compete for all elected offices in regular elections. Although these elections clearly fell below the minimum standards of democracy, they were more than hollow rituals. Yet despite meaningful competition, challengers remained undercompetitive until the 1990s because they made niche-oriented appeals to minority electoral constituencies. As a result, challengers ceded the broad center and electoral majorities to the incumbent, thus allowing dominant party rule to remain in equilibrium for most of the 20th century.

Equilibrium dominance – the long-term continuous rule of a single party with existing but ineffective challenger parties – should not have occurred in Mexico according to existing theory. Mexico had a sufficiently permissive electoral formula, enough social cleavages, a high enough level of economic development, and enough voters disapproved of the PRI's performance in office that at least one other party should have been fully competitive. The spatial dynamics of party position-taking in models that assume no incumbency advantage (i.e., so-called neutral theories) also predict at least two competitive parties. A more specific version of this spatial theory that was crafted for Mexico logically implies the same conclusion.