The Bulgarian Social Democratic Party (BSDP) was an offshoot of the movement that produced the BCP. The main socialist party in Bulgaria between the world wars, the BSDP was disbanded by the communists in 1948. It resurfaced in 1990, resuming its advocacy of government reform and elimination of social privilege. The BSDP saw a freely elected National Assembly as the chief instrument of popular democracy. The BSDP party platform also called for close economic ties with Europe, disarmament, and respect for private property. The BSDP was a founding member of the UDF and, under the controversial leadership of Petur Dertliev, one of its most active participants.

The history of the BSDP followed closely that of the communists, except that the latter had a larger following. The BSDP recovered official status in 1990 after being disbanded in 1948. Representing the middle class, the party stood for private property rights, a multiparty parliamentary system of government, radical reduction of the military budget, and active participation in the European Community. Membership in 1991 was 25,000 to 30,000.

The Petkov branch of the Bulgarian Agrarian National Union (BANU), the third of the prewar parties to emerge as an independent entity after Zhivkov, was the part of the agrarian movement that had actively opposed the communists between 1944 and 1947 and thus did not survive the postwar communist consolidation. The "official" BANU, showpiece opposition party to the BCP from 1947 until 1989, also was revitalized in 1990. In 1990 and 1991, efforts were made to reunite the two factions. (Petkov himself was officially rehabilitated by the National Assembly in 1990.) In its new incarnation, the Petkov branch advocated complete government decentralization, extensive support for agricultural privatization and investment, punishment of the communists and "official" agrarians for crimes against the Petkov branch, and a general return to the populist ideas of Stamboliiski (see Stamboliiski and Agrarian Reform , ch. 1). Together with the BSDP, the Petkov BANU was the largest (110,000 members in 1991) and most active constituent of the UDF.

Data as of June 1992