During the 1920s, he opposed Joseph Stalin's bureaucratization and centralization of the party and advocacy of "socialism in one country", becoming linked to Leon Trotsky and a leader of the Left Opposition movement.

[3] He subsequently left the town to attend the classically oriented gymnasium in the provincial capital of Oryol, where Preobrazhensky remembered himself as the "second-best student in the class".

[6] That summer, upon his return to the family home at Bolkhov, Preobrazhensky closely reviewed this and other illegal material and determined to become actively involved in the revolutionary movement seeking the overthrow of the Tsarist regime in Russia.

[7] Preobrazhensky decided to henceforth "devote a minimum of time to the gymnasium's subjects", merely enough to attain passing marks, to dedicate the bulk of his hours to the study of history and economics.

Together with two friends, Evgraf Litkens and Ivan Anisimov (who later joined the Mensheviks), Preobrazhensky declared his formal allegiance to the RSDLP late in 1903.

[9] During the summer before his eighth and final year at the Orël gymnasium, Preobrazhensky worked as a RSDLP propagandist to the workers of the Dyatkovo factory in Bryansky raion.

Preobrazhensky was able to recruit the son of the Bryansky police to the RSDLP and successfully managed to conceal his small rotary mimeograph machine from searching authorities in a locked drawer of the inspector's own desk.

[10] In November 1905, Preobrazhensky traveled to Moscow where he was promoted to the position of chief propagandist for the urban Presnensky raion.,[11] and for the next 12 years, he was an itinerant professional revolutionary.

[13] It was at this time that Preobrazhensky became closely affiliated with Nikolai Bukharin, himself a popular Left Communist leader and member of the party Central Committee.

Preobrazhensky was elected a full member of the Central Committee of the Russian Communist Party at its 9th Congress, which opened at the end of March 1920.

During 1920–21, the staff employed by the secretariat expanded to over 600, including a new section tasked with building up a card index that graded party members according to whether they were 'active and reliable', 'promising', or 'rank and file'.

In 1921, Preobrazhensky was appointed President of the party's Financial Committee and Chief of the Directorate for Professional Training in the People's Commissariat of Education.

He was the main author and leader signatory of The Declaration of 46, which called for greater freedom of dissent within the communist party, and attacked the leadership for having no strategy to deal with the current economic crisis.

Preobrazhensky visited Trotsky when he was in Berlin for medical treatment in 1926, an "interesting" fact that Stalin, noted in a letter to Molotov after it had been reported back to him by the Ogpu.

[18] Preobrazhensky (and Trotsky) advocated for a rapid pace of industrialization in the context of the Soviet Union's New Economic Policy, arguing that the numerically limited Communist Party faced a grave danger of being swamped by the richest and most powerful individuals in the villages (the so-called kulaks) and the mass of peasants who might naturally follow these local leaders.

[19] This program was presented polemically in opposition to the policy of the Communist Party leadership, headed in this period by Stalin and Preobrazhensky's former collaborator on the book The ABC of Communism, Nikolai Bukharin,[19] who felt the rich peasantry to be under control and who advocated reducing prices and improving quality of textiles and manufactured goods to spur peasant production of grain and win the sympathy of the rural and urban working people for the task of socialist development.

[21]Victor Serge, who met Preobrazhensky in the 1920s, recalled that "he had driven himself so hard that during meetings it seemed that he might at any moment drop off to sleep; but his brain was still fresh, and crammed with statistics.

"[22] In 1927, the United Opposition, which now included former foes, Grigory Zinoviev, Lev Kamenev and their supporters, thrashed out a comprehensive Platform summing up their criticisms of the party line (which Trotsky published while he was in exile, under the title The Real Situation in Russia).

The presence of the 'Wrangel officer' was given huge publicity in the soviet press, to discredit the left, though when the Central Committee met in October, Stalin casually admitted that he was an OGPU informer.

[24] When their case came before the Central Control Commission, Preobrazhensky and Serebryakov submitted a statement seeking to refute the slander about a 'Wrangel officer', in which they admitted to a share of responsibility for the existence of the press.

On an unknown date, he joined Ivan Smirnov's secret opposition group, which later by the end of 1932 entered a bloc with Leon Trotsky and some others in the USSR.

[31] He was arrested a second time on December 20, 1936, but unlike his old comrades, such as Serebryakov, Mrachkovsky, Smirnov and Ter-Vaganyan, he was not a defendant at any of the Moscow Show Trials, though he must have been under the same pressure as they all were to make a false confession.

Robert Conquest the historian of the Great Purge wrote that: So much more admirable, then, is the sense of truth and personal courage shown by men like Preobrazhensky, who even under the pressure of such arguments seems to have 'died in silence'.