Democratic socialists are either proletarians who are not yet sufficiently clear about the conditions of the liberation of their class, or they are representatives of the petty bourgeoisie, a class which, prior to the achievement of democracy and the socialist measures to which it gives rise, has many interests in common with the proletariat. It follows that, in moments of action, the communists will have to come to an understanding with these democratic socialists, and in general to follow as far as possible a common policy with them – provided that these socialists do not enter into the service of the ruling bourgeoisie and attack the communists. It is clear that this form of co-operation in action does not exclude the discussion of differences.
Friedrich Engels