The two-party structure began to decline in the early twentieth century as leftist parties emerged and the country experienced a quarter-century of political instability. Ecuador had at least four communist and socialist parties. The oldest was the Ecuadorian Socialist Party (Partido Socialista Ecuatoriano--PSE), founded in 1925 as a section of the Communist International. Consisting of a small group of intellectuals, the PSE was influential only through coalitions either with groups on the left, including the Communists, or more often, with the PLR. The PSE was one of the few parties that was neither regionally based nor personalist in character. Although it depended on wealthy groups and individuals for support, the PSE played a major role in formulating social welfare legislation.

The PSE gave birth to both the Moscow-oriented Ecuadorian Communist Party (Partido Comunista Ecuatoriano--PCE), which broke away in 1928, and the pro-Cuban Revolutionary Socialist Party of Ecuador (Partido Socialista Revolucionario del Ecuador--PSRE), which broke away in 1962. The PCE, a legal party, generally has concentrated on enhancing its position within organized labor, student organizations, and the educational bureaucracy it had little voter appeal. By the 1970s, the PSRE had become the strongest advocate of revolution in the country. The PSRE and PCE, along with Christian leftists and Maoists, joined in 1977 to form a Moscow-line leftist front called the Broad Left Front (Frente Amplio de la Izquierda--FADI). Another PCE splinter group, the proChinese Communist Party of Ecuador--Marxist-Leninist (Partido Comunista del Ecuador--Marxista-Leninista--PCE-ML) formed in 1972.

Several noncommunist and Christian Democratic parties also emerged in the twentieth century. The Ecuadorian Nationalist Revolutionary Action (Acción Revolucionaria Nacionalista Ecuatoriana--ARNE), founded in 1942, was a highly nationalistic, anticommunist, quasi-fascist group with its strongest appeal among youths in the Sierra. The center-right Social Christian Party (Partido Social Cristiano--PSC) was established in 1951 and became the ruling party when Febres Cordero assumed the presidency in 1984. The Christian Democratic Party (Partido Demócrata Cristiano-- PDC), founded in 1964, affiliated with the International Christian Democratic Association. Its center-left platform attracted a small but growing following among workers, students, and young professionals.

In 1970 Rodrigo Borja broke away from the PLR and formed, in 1977, a Quito-based Social Democratic party, the center-left Democratic Left (Izquierda Democrática--ID). The ID became Ecuador's largest party and the voice of a new generation of reformist, professionally trained political leaders. The Alfarist Radical Front (Frente Radical Alfarista--FRA), a populist and centrist party, was established in 1972. Popular Democracy (Democracia Popular--DP), an affiliate of the Christian Democratic International, was founded in 1978 as a coalition of the PDC and the Progressive Conservative Party (Partido Conservador Progresista--PCP) and a breakaway faction of the18e he PC. Because of its Christian Democratic membership, DP often was hyphenated with Democracia Cristiana.

Data as of 1989