China will amend its constitution to allow Presidents to serve more than two terms, the state-run news agency Xinhua announced Sunday. The change is momentous because it confirms Xi Jinping has become the country’s most powerful leader since Mao Zedong. He is abolishing term-limit rules and other norms that Deng Xiaoping created in the 1980s to prevent a repeat of Mao’s disastrous rule.

After taking power in 2012, Mr. Xi used an anticorruption campaign to purge rivals and concentrate power in his hands, breaking the post-Mao...