It is a bit surprising to see Kant of using the term "human nature" in his moral theory, at least as Rawls interprets him. However, given his awareness of the inner difficulty of his formerly Manichean moral psychology, i.e. the almost hopeless difficulty in connecting the moral law from the intelligible world and the real human beings in the sensible world, say, the difficulty of motivation, his move to an Augustinian moral psychology is understandable. The gap inherent in the earlier dualism seems unbridgeable. In essence it is the gap between two separate selves. In the later Augustinian moral psychology, by contrast, there is only one self, though possessed with conflicting predispositions, one of them pertaining to the moral law. It is with this united self that Kant observes (assigns) a human nature according to which "we cannot help but be moved to identify with" the moral ideal informed by the moral law.
We put aside for the moment whether Kant observes or expects (assigns) such a human nature to the human beings in the sensible world. Instead we examine what kind of human nature it looks like. The very brief depiction "cannot help" reveals an inner tendency for us to elevate ourselves to the ideal level described by e.g. the kingdom of ends. It's like the longing for becoming angels. This is connected to the idea that humans are fallen angels and thus it is in our nature to tend to go back, to the realm of heaven or at least to the community of moral saints.
However, Hume and Hobbes depict a very different human nature, which is more and more confirmed by modern sciences, especially by recent developments in empirical psychology and anthropology, informed by evolutionary biology and neuroscience, as well as human strategy studies, i.e. game theory and rational choice theory. In this picture, human nature is not that of fallen angels but of ripe apes. Then the challenge to Kant's conception of human nature is not only about the attainability of the ideal, but more of its desirability. For the Humean human, the ultimate motivation is not to get elevated to angel or saint, but to stay in safe distance from beastly passions.
