Trotsky’s call was directed at the leaders of the Communist Party, who instead of seeking a united front with the socialists, were denouncing the socialists as “social fascists.”
While intended to achieve the class-unity which was necessary to defeat fascism, as Trotsky saw it, the united front did not include the parties ceasing criticism of one another. In fact, in order to get the best policy, it was essential that the two parties continued to criticise each other and compete for the loyalty of the workers.
Further, the united front was to be formed by a public agreement between the leaders of the two parties, who thereby recognised each other’s legitimacy, even while disagreeing with each other. This differs from the idea of a ‘Red United Front from Below’ put forward by Stalin, which meant denouncing the leaders of the opposing party as ‘social fascists’ but calling upon those who supported the social democrats to form a united front ‘behind the backs’ of their own leaders, ‘from below’.