They call Kant a whitewashing spokesman for the German middle class: "The characteristic form which French liberalism, based on real class interests, assumed in Germany we find again in Kant. Neither he, nor the German middle class, whose whitewashing spokesman he was, noticed that these theoretical ideas of the bourgeoisie had as their basis material interests and a will that was conditioned and determined by the material relations of production. Kant, therefore, separated this theoretical expression from the interests which it expressed; he made the materially motivated determinations of the will of the French bourgeois into pure self-determinations of “free will”, of the will in and for itself, of the human will, and so converted it into purely ideological conceptual determinations and moral postulates. Hence the German petty bourgeois recoiled in horror from the practice of this energetic bourgeois liberalism as soon as this practice showed itself, both in the Reign of Terror and In shameless bourgeois profit-making."
But they are not always negative on Kant. For instance, in the Preface to Socialism, Utopian and Scientific, Engels gives credit to Kant for being the founder of German idealism of which Marxism is an offshoot. "We German socialists are proud that we trace our descent not only from Saint Simon, Fourier and Owen, but also from Kant, Fichte and Hegel.”