"Until the accesion of the liberal reform Communist Mikhail Gorbachev to the leadership of the Communist Party of the Soviet Union in 1985, Zhivkov was grovellingly obsequious towards each of the successive rulers of the Soviet Union, and Russians and Bulgarians began jokingly to refer to Bulgaria as the 'sixteenth republic' of the Soviet Union. What they did not know for sure was that on two occasions, once to Khrushchev and once to his successor Leonid Brezhnev, Zhivkov proposed in all seriousness that Bulgaria should formally join the Soviet Union." - The Balkans: A Post-Communist History (2007) p. 88.
"Since the early 1950s Bulgaria was beyond any doubt the closest Soviet satellite in Eastern Europe, and the Communist regime was perhaps the most stable in the region. The 'total integration with USSR' doctrine promoted by the ruling party elite reached an apogee in the discussions held in 1961-62 at the politburo level, considering possible political unification and Bulgaria's application to join the USSR." - The Roundtable Talks and the Breakdown of Communism (1996) p. 179.
"Although there as never any doubt concerning Zhivkov's loyalty to the Soviet Union, the suggestions made in the 1970s that Bulgaria join the USSR as another federal republic were dropped in the 1980s. In any case, Zhivkov was no enemy of Bulgarian nationalism, which he promoted strongly during the 1970s and after." - The Soviet Union and Eastern Europe (1986) p. 34.