TOKYO—Many of Japan's political and intellectual leaders remain committed to nuclear power even as Japanese public opinion has turned sharply against it. One argument in favor rarely gets a public airing: Japan needs to maintain its technical ability to make nuclear bombs.

"I don't think Japan needs to possess nuclear weapons, but it's important to maintain our commercial reactors because it would allow us to produce a nuclear warhead in a short amount of time," Shigeru Ishiba, a former defense minister, said in...