As Portugal became democratic after 1974, it also developed a political party system with a full spectrum of parties that ranged from the far left to the far right. During the SalazarCaetano regime, only one party was legal, the National Union (União Nacional--UN), later renamed the National Popular Action (Acção Nacional Popular--ANP). The UN/ANP was dissolved in the first weeks of the revolution, and a great variety of new parties soon replaced it.

Some political parties emerged very quickly because they already existed in preliminary form. Several factions of the old UN/ANP, for example, became separate political parties after the revolution. The socialists and, to a far greater extent, the communists already had underground groups operating in Portugal, as well as organizations in exile. Finally, some opposition elements had formed "study groups" that served as the basis of later political parties.

The party system increased in importance during the Second Republic. Large, strong parties were fostered under the d'Hondt method of proportional representation, and parties soon began to receive state subsidies. The parties' strength was also bolstered by their exclusive right to nominate political candidates and by the strict party discipline they enforced on successful candidates once they entered parliament. By the beginning of the early 1990s, only four parties regularly won seats in the parliament, and two were so much stronger than the others that Portugal seemed on the way to an essentially two-party system.

Data as of January 1993