To what extent was Stalin's rise to power due to the weaknesses of his opponents?

When in the planning stages it is important to pick out the key phrases from the question, so that you can give the most effective answer possible. The most important part of this particular question is 'to what extent' - the weaknesses of Stalin's opponents were a factor, but were they the most crucial? How much of an impact did these weaknesses have in comparison to other factors, such as Stalin's position as General Secretary, the policies surrounding the leadership debate, Stalin's position within the Party following Lenin's death, and so on? It is also important to note that the question refers to opponents, plural, so you would need to look at Zinoviev, Kamenev, Bukharin and so forth, not just Trotsky, although he was arguably Stalin's main rival.A good answer to this question would consider the weaknesses of Stalin's opponents first, perhaps with reference to Lenin's 'Letter to the Party Congress' where he outlined his feelings on the Party members closest to him, before examining other factors. The different personalities of the competitors and their relationship to Lenin played an important part, as well as their stance on key issues that would affect the Party going forward. Some of them, such as Trotsky, were seen as too vocal and a risk to Party unity, while Stalin changed his theories according to who he needed to get rid of next and never enforced his own ideas, adopting those most in favour with the Party. The relationships between the competitors is also an important factor - Zinoviev and Kamenev stopped Lenin's Testament being read at the 13th Party Congress, for example, as they feared it would help Trotsky, assuming Stalin to be their ally against him. The historian J. N. Westwood stated that Stalin 'could stand back and watch his rivals dig their graves, occasionally offering his spade to one or other of them' - although his opponents had their personal failings and collectively underestimated his grip on power, Stalin's own opportunism and political outmanoeuvring are also important factors to consider.¹J. N. Westwood, Endurance and Endeavour: Russian History 1812-1971, (Oxford University Press: 1973), p. 287.