The UDD was established in 1955 by a merger of the Dahomeyan Progressive Union (UPD) and the African People's Bloc (BPA).
[3] The party failed to win a seat in the French National Assembly in the 1956 elections, and split into two factions later in the year over the issue of affiliation with the African Democratic Rally (RDA); former BPA leader Justin Ahomadégbé-Tomêtin became head of the dominant RDA faction, whilst Alexandre Adandé and former UPD leader Émile Derlin Zinsou headed the smaller UDD-Convention.
[3] Despite its internal problems and regionalised support base, the UDD emerged as the second-largest party in the 1957 Territorial Assembly elections, winning seven seats.
[4][5] Following the elections, accusations of fraud led to an agreement being reached with the PRD to redistribute nine seats.
[4] In elections held the following year, the Dahomeyan Unity Party, a merger of the PRD and the RDD, won every seat after President Hubert Maga changed the electoral system,[6] leaving the UDD without parliamentary representation.