The hottest politician in Japan these days — and one of the biggest threats to Prime Minister Yoshihiko Noda — is Osaka Mayor Toru Hashimoto. As Mr. Noda appears to be moving closer to trying to restart idled nuclear reactors, Mr. Hashimoto is becoming more vocal in opposition. That could complicate Mr. Noda's nuclear policy, and possibly frame the next general election.

With most top politicians in Nagatacho dancing around the delicate question of whether to endorse restarting nuclear energy, Mr. Hashimoto openly opposes...