Why would the Japanese want to spend billion of dollars in building nuclear weapons, when the US is already paying the bill in defending them?
Besides, one should be careful with the words of a "conservative Japanese political leader." Wonder what they were saying before Pearl Harbor?
The problem with relying on the United States to nuke China in order to save Japan, is that under the doctrine of mutual assured destruction, nuclear war is considered to be suicidal. Therefore the U.S. would have to be willing to suffer casualties from Chinese nukes in order to save another country. If the Chinese government doubts our willingness to do this, they would not be deterred from attacking Japan. In contrast, China would be less likely to doubt the willingness of Japanese to defend their own country.
This same situation exists with regards to Taiwan.