Why was Stalin able to acquire and maintain total domination over the party organisation?

Stalin's rise to power is remarkable in a number of senses, not least of which is the fact that with the long list of potential successors to Lenin who had been involved in the October 1917 Bolshevik revolution, Stalin was not the most prominent figure that one would have considered a logical replacement for Lenin. Although one of the central figures in the Bolshevik party, he was not held in the same stature as Lenin, Trotsky, Zinoviev, Kamenev, and Bukharin by other communists throughout Russia. It is thus all the more interesting to note how he managed to use his semi-obscurity to his advantage in gaining control of the party.
At the time of the revolution, Russia was a country dominated by peasants; there was little industry, and no unified, working-class proletariat at all. By 1921, despite the ending of the war, the economy had completely collapsed, and it was necessary for the communists to take some action to get the economy back on its feet. This came in the form of the New Economic Programme (or NEP), which involved a mixture of state and private ownership, private agriculture, and attempts to rebuild the terms of trade between the town and the country.
However, with the stroke suffered by Lenin in December 1922, and his death in January 1924, the struggle to take his place resulted in the eventual dismantling of the NEP. With Lenin's death, a joint leadership consisting of Kamenev, Zinoviev, and Stalin was formed. At this time, Stalin held the position of Secretary of the party (then much less important then it was to become later). These three were supported by Bukharin; this group aligned themselves against Trotsky, whom they saw as a potential Napoleon in the making. Even on his sick bed, Lenin had seen the dangers ahead, and had published a critique of all those who were involved in the power struggle. This document became known as his testament, and in it he expressed his doubts over Stalin's emerging ruthless personality, and even recommended that the other members of the group eject Stalin from his position as party Secretary. Given that the testament had criticisms of each of the individuals, they collectively decided to put it away, and get on with the business of ruling.
Even before Lenin's death, Trotsky and other left-wing opponents had begun to raise doubts over the NEP, and were arguing for a policy shift. They believed that investment in the capital goods sector was needed to enable the economy to provide the goods that the peasants demanded, and that this investment could only be afforded with a tax on the peasants. Extracting resources from the peasants would be combined with the gradual introduction of collectivised large-scale farms, that would be more efficient than small-scale private farming. This opposition view was not popular, and made little headway at the time, with the prevailing view being that investment was needed throughout the whole economy, and not just in the industrial sector. With the expansion of the agricultural sector at the same time as the industrial sector, there would be production on farms of the food needed for the workers in the towns, and the raw materials needed by the industrial sector.
It was soon after Lenin's death that Stalin began to strengthen his position in the group of three by undercutting his rivals; he was able to use his powers of appointment to reduce the power base of Kamenev and Zinoviev, and began to launch subtle attacks on them. This forced them to distance themselves from Stalin; they moved to the left and began questioning the NEP in order to achieve this. The divisions were brought to a head at the Fourteenth Party Congress in December 1925, when Kamenev ended a long speech with a personal attack on Stalin, which provoked a split between Stalin's supporters and those loyal to the party above any individual. Stalin won a huge victory in a vote on the leadership, and had forced his two rivals into opposition. They tried to form an alliance with the already-defeated Trotsky, but it was too late, as they were both eventually removed from their important positions in the party, and were thus left impotent.
Although the NEP was by now having the effect of increasing harvest yields, consumer goods were in short supply. Stalin reacted to what he believed to be hoarding of grain by the "kulaks," or wealthy peasants, who were holding the towns to ransom by their actions, by ordering troops into the villages in order to seize the grain by force. When censured by the Politburo for his actions, he justified them by arguing that through his actions he could increase the standard of living for the poorer peasants, and provide the food and raw material for the towns. This change of stance marked a divergence between Stalin and Bukharin's positions, and this time it was the latter whom Stalin had managed to isolate by taking over the institutional sources of supports of the right. He had Bukharin and his supporters removed from the Politburo, and the rout of the Right was complete - Stalin now emerged as undisputed emperor of Russia by the end of 1929. He had successfully divided, and then conquered his enemies. He was now free to abandon the NEP, and put in place policies of collectivisation and industrialisation, contained in the first Five Year Plan, along with the introduction of mass terror.
Although there is a debate as to whether the structure of the Communist party was such that it made the emergence of a dictatorial one-man rule inevitable, there is also the issue of whether it was Stalin in particular who could have assumed such power and become leader. Examining the events leading to his emergence shows a complex interaction of various factors that contributed to his victory. First of these was his personal characteristics, which as we have seen showed him to possess not only the political and oratorical skills of his opponents (although perhaps not quite in the same division as them), but also his ruthlessness and Machiavellian lust for power for his own purposes. Secondly, it was his strategic position in the Secretariat that allowed him to undermine the political bases of his opponents by placing his allies in key positions among the party. Thirdly, he showed skill in dealing with his opponents in piecemeal fashion, judging precisely how and when to isolate them and label them as "deviations." Finally, it has to be said that many of his opponents displayed a lack of political awareness that only hastened their political downfall.
The question is how much importance should be placed on each of these factors. The dominant view in the 1950s was that Stalin's victory could in a large part be explained by Stalin's position as General Secretary, and organisational skill that incorporated his supporters in key positions, which made Stalin himself almost invulnerable to attack. This argument also includes the failures of the Party structure, which in its desire to see a disciplined party apparatus above all, allowed a circular flow of power to emerge. With the party congress choosing the Central Committee, which in turn chose the Politburo and the General Secretary, the man who ran the party apparatus that chose delegates for the congress (i.e. the General Secretary) could build a machine by placing officials in positions to control the entire system, from the selection of delegates to the congress, to the expansion of the Central Committee to ensure the Politburo was composed of members that he desired.
More recent historians suggest that these factors have been exaggerated, and although important, they do not offer an entire explanation on their own, contrary to what the 1950's observers believed. Instead, more consideration should be placed on Stalin's ability to present a politically persuasive program and convince the party to accept it. Given the increasing numbers of economic administrators on the Central Committee who were enthusiastic about Stalin's programme of industrialisation, it could be argued that this was as much a part of the explanation for Stalin's rise to dominance as the organisation of the party. By creating a clique of twenty or thirty influential members of the Central Committee who were resolute supporters of his programme, Stalin could be assured of victory in whatever policies he was considering.
This argument is strengthened by considering the premise that it is necessary to have a sound political programme to support the strategic ability to build a political machine to one's own advantage. Even the strongest political machine rests upon the self-interest and loyalty of the leader's subordinates, and without these it could collapse, so it was necessary for Stalin to have an appealing political programme to cement his position after he had achieved power; in order to take advantage of the "circle of power," he would have to find enough supporters to place in the key positions, which would require an appealing programme. Without this, it would have been difficult to manipulate the party, and thus we may deduce that the populist programme and organisational skill were both significant in different ways; the former to attract important supporters, and the latter to place them in important positions. It is difficult to label either factor as more important. One needs an appealing programme, but it could come either before the seizure of power (which would facilitate the gaining of power by manipulation of the party apparatus), or afterwards to secure the position (by eliminating opposition and building more loyal and hardened support within the party). However, you cannot have one without the other; Stalin did have both, and it seems he was able to use both simultaneously to build support and remove opposition at the same time, which combines both of the above arguments as being important in his rise to power.
Finally, there is the issue of terror, which allowed Stalin to dominate both the party and the country for the rest of his rule. This terror continued right up until his death, although at a reduced level than under the initial phase known as the Great Purge. To show how terror was applied to the party, we need only examine the changes between the Seventeenth Party Congress in 1934 and the Eighteenth in 1939. Of the 2,000 delegates present in 1934, only 59 remained five years later. In the interim, over 1,000 had been arrested (and the majority were shot), while 98 of the 149 members of the Central Committee elected at the Seventeenth Congress had been arrested and shot. Anyone seen as a threat to Stalin ended up dead, either through arrest and show trial (e.g. Kamenev, Zinoviev, and Bukharin), or assassination (Kirov and Trotsky). There was also a campaign that decimated the upper echelons of the military. The need to protect the state against its enemies became a cover for getting rid of anyone who appeared to deviate in any way that Stalin perceived as a threat. Up to 20 million people were killed. The threat of arbitrary arrest, torture, exile to the gulags in Siberia, or execution ensured that Stalin was easily able to maintain the power he had gained, and thus he remained in power until his death in 1953.