There are four ways in which the Soviet Union can change its leader: the death of the incumbent; his incapacitation through illness; his resignation; or his enforced removal. Each has different political implications and each is quite possible. In the course of the USSR's existance since 1917, there have only been four undisputed leaders: Lenin, Stalin, Khrushchev and Brezhnev. This leaves out Malenkov, who was a key secretary of the Central Committee ( CC) of the Communist Party of the Soviet Union ( CPSU) for just a week after Stalin's death in March 1953 and was chairman of the USSR Council of Ministers for almost two years. Lenin, Stalin and Brezhnev died and Khrushchev fell victim to a successful coup.
It is now a commonplace that the Soviet leader occupies the post of Secretary General of the CPSU and that his main power base is the party apparatus. (Between 1952 and 1966 the party leader was called the First Secretary.) However before 1953 this was not the case. The first, and arguably the only leader with charisma, Vladimir Ilich Lenin, was not even a secretary of the communist party. His main office was that of chairman of the Council of People's Commissars (Sovnarkom), the post he preferred since he was enamoured of administration. Of course he was a member of the highest party organ, the CC ( 1917-19), and then the Politburo, but his power and authority did not derive from any office, it was based on his intellectual dominance and the fact that he had led the Bolsheviks to victory in October 1917. Although his cabinet was Sovnarkom, members could appeal over his head to the Politburo. Hence, when he suffered his first stroke in 1922, it was not clear which institution was stronger. However he was disturbed by the Politburo's accumulation of responsibilities, many of which he regarded as the legitimate tasks of government. Undoubtedly, he would have reversed this trend had he been physically able to do so.
When Lenin died it was not clear whether the head of government was the new leader. A collective leadership came into being until Stalin was able to achieve primacy by 1929. He was Secretary General of the CPSU between 1922 and 1934 and afterwards a secretary until his death. He was also Prime Minister between 1941 and 1953. The basis of