With the death of Lenin in 1924, it seems likely that Trotsky would become the next leader of the Communist Party as he was Lenin’s right hand man.

Yet while Trotsky was a brilliant leader, he was not popular with the party due to his wealthy Jewish background and his late conversion to the Bolshevik cause (he was a Menshevik before)

The Mensheviks were the result of a split in the Russian Social Democratic Labour Party.  They had the larger support base, and was the more moderate of the two factions (the other being the Bolsheviks).  Yet, despite the division the two were deeply committed to Marxist principles.

He also displayed a lack of commitment to the party, and was very arrogant.  This all provided Stalin with a lot of ammunition to use against Trotsky.