The country has been operating under single-party system since 1965 when opposition parties were abolished.
The party was revived after the end of the Communist single-party system in 1989.
In 1946 it became a Socialist state with a single-party system.
Within a year, the single-party system was abolished, and the first free elections were being prepared.
Existing political parties were banned; after a period of military rule a single-party system was established in 1969.
In single-party systems, one political party is legally allowed to hold effective power.
In the latter case, the definition between Dominant and single-party system becomes rather blurred.
But it was at least as serious a test for China's single-party political system.
"For many years Mexico lived under a virtual single-party system," he said in an interview.
This would help stop the move towards the single-party system, which poses a threat to the respect for human rights.