Pt. 1
By aligning with certain factions, than others, and building a "united front" against Trotsky, accusing him of factionalism and division. From Stalin's
what reasons did Trotsky have for contrasting the "old ones," who may degenerate, to the "youth," the Party's "truest barometer"; for contrasting the "old guard," who may become bureaucratic, to the "young guard," which must "capture the revolutionary formulas by storm"? What grounds had he for drawing this contrast, and what did he need it for? Have not the youth and the old guard always** marched in a united front** against internal and external enemies? [...] Trotsky is in a bloc with the Democratic Centralists and with a section of the "Left" Communists—therein lies the political significance of Trotsky's action. - JV Stalin, 1923
Hence, the Trotskyites' theory of the division of Leninism into two parts: pre-war Leninism, the "old," "useless" Leninism with its idea of the dictatorship of the proletariat and peasantry, and the new, post-war, October Leninism, which they count on adapting to the requirements of Trotskyism. Trotskyism needs this theory of the division of Leninism as a first, more or less "acceptable" step that is necessary to facilitate further steps in its struggle against Leninism.
The post-October part of the history of our Party, however, is real, genuine history. In the former, there are the "old," "pre-historic," unimportant cadres of our Party. In the latter there is the new, real, "historic" Party. It scarcely needs proof that this singular scheme of the history of the Party is a scheme to disrupt the unity between the old and the new cadres of our Party, a scheme to destroy the Bolshevik Party principle.
These 3 passages in 1923 align Stalin, ironically, with the "old Bolsheviks" he later purged, who feared Trotsky's revolutionary and ambitious revisions to Leninism and being dominated by Trotsky. He was "picking off" Trotsky by mobilizing a silent majority of old cadres against him.
In the 1923-1927 period, Stalin aligns with rightists such as Bukharin, and other anti-Trotsky figures like Kamenev and Zinoviev, being a public voice for the center himself. He keeps Trotsky from ascending to primacy in the party.
By 1927, the alliances have reversed. Stalin has used his position as General Secretary, comrade card index, the promoter and human resources man of the party, to promote and cultivate new allies in the party. He is now the "mainstream", and Kamenev, Trotsky, and Zinoviev form the "left opposition", and criticize Stalin for suppressing Lenin's will. Stalin responds ingeniously in his speeches, and instead cherry picks Lenin's will, claiming it said terrible things about Trotsky, Kamenev, and Zinoviev.
Here is the relevant passage in the "will": "I shall not go on to characterise the personal qualities of the other members of the Central Committee. I shall merely remind you that the October episode with Zinoviev and Kamenev was, of course, not accidental, but that they can be blamed for it personally as little as Trotsky can be blamed for his non-Bolshevism." - Stalin, 1927
He begins labelling his enemies the "opposition", and claims the rest of the party is non-factional.
Firstly, because the Central Committee did not want and had no right to legalise Trotsky's faction, or any factional group. In the Tenth Congress resolution "On Unity," Lenin said that the existence of a "platform" is one of the principal signs of factionalism.
Then, he accuses Trotsky of feeding information to the West:
When members of the Central Committee like Trotsky and Zinoviev supply false reports about our Party to the imperialists of all countries, assuring them that we are ready to make the utmost concessions, including the abolition of the monopoly of foreign trade, it can have only one meaning: Messieurs the bourgeois, press harder on the Bolshevik Party, threaten to go to war against them; the Bolsheviks will agree to every concession if you press hard enough.
By 1936, Stalin's overall strategy has worked. To summarize, he:
Built a following in the center by criticizing the feared Trotsky and winning over comrades with promises
Framed Trotsky, his earlier allies, and his later allies, as factionalists
Stole Lenin's legacy and claimed Trotsky was out to destroy Leninism and construct Trotskyism out of his own vainglory
Doled out favors and promotions from his position to build support
Lapped accusations against Trotsky including collaboration with capitalists