It is certainly not based on formal ideological affinity. Indeed, it involves sweeping under the carpet the official ideological differences. The basis of the relationship has been primarily economic, what each side as its economic interests of the next 20-30 years, if not longer. It will not be easy. And since a triadic competition usually reduces to a dyad, it is possible to foresee a U.S.-Japan economic arrangement in opposition to western Europe.
China offers all four in one fell swoop.
It seems elementary that the U.S. would therefore give priority to including China in some zonal arrangement. This will be of course in Japan's interest as well, if not ever more. But given Japan's legacy of Chinese resentment, the U.S. must necessarily take the political lead in trying to bring this about. China has learned from its history that it can only be respected in the world if it is a unified state. The underlying political strength of the Chinese Communist Party resides in the fact that it restored such unification in 1949 after a long period of disintegration.