More worried about China than Russia/Soviet Union on that. Heck, even the novel version of 2001 A Space Odyssey, the Communist Chinese were a bigger threat in space than the Communist Russians. I think even in Clarke's other mega-series Rama, it was the same thing.
And since Clarke worked with a newborn NASA, he probably knew more about the US and the enemies at the time and what everyone had in mind for space in the long term.
